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TWENTIETH   CENTURY  TEXT-BOOKS 

EDITED    BY 

A.   F.   NIGHTINGALE,    PH.D.,   LL.D. 

SUPERINTENDENT    OF    SCHOOLS,    COOK    COUNTY,    ILLINOIS 


- 


TWENTIETH   CENTURY   TEXT-BOOKS 


ANIMALS 


A  TEXT-BOOK  OF  ZOOLOGY 


BY 

DAVID  S.  JORDAN,  M.  S.,  M.  D.,  PH.  D.,  LL.  D. 

PRESIDENT   OF   LELAND   STANFORD  JUNIOR   UNIVERSITY 

VERNON  L.  KELLOGG,  M.  S. 

PROFESSOR   OF    ENTOMOLOGY    IN    LELAND    STANFORD   JUNIOR    UNIVERSITY 

AND 

HAROLD  HEATH,  PH.D. 

PROFESSOR   OF   ZOOLOGY   IN   LELAND   STANFORD  JUNIOR   UNIVERSITY 


OF  THE 

UNIVERSITY 

OF 


NEW     YORK 

D.     APPLETON     AND     COMPANY 
1907 


BIOLOQY 

LIBRARY 

G 


CoPtRIGHT,    1902 

BY  D.    APPLETON  AND  COMPANY 


Published  May,  J902 


ANIMALS 

A  TEXT-BOOK  OF  ZOOLOGY 


PUBLISHERS'  NOTE 

THIS  volume  comprises  Animal  Life  and  Animal  Forms, 
each  of  which  is  designed  for  schools  that  assign  but  a  half 
year  to  Zoology.  The  two  are  issued  in  this  form  to  meet 
more  satisfactorily  the  requirements  of  normal  schools  and 
all  higher  schools  that  give  a  full  year  to  the  subject.  It 
is  believed  that  it  will  be  found  a  very  desirable  text,  also, 
for  an  elementary  course  in  colleges. 

The  prefaces  of  the  two  sections  of  the  book  explain 
the  plan,  scope,  and  purpose  of  each.  If  preferred,  the 
second  part  of  this  volume,  Animal  Forms,  treating  the 
morphological  side  of  the  subject,  can  be  first  taken  up ;  or 
portions  of  each  may  be  chosen  and  arranged  in  any  order 
that  may  best  suit  the  teacher's  purposes.  The  first  part, 
Animal  Life,  gives  the  ecological  features  of  the  subject 
special  prominence. 

Full  indexes  will  be  found  at  the  end  of  each  section. 


TWENTIETH   CENTURY  TEXT-BOOKS 


ANIMAL    LIFE 


A  FIRST  BOOK  OF  ZOOLOGY 


BY 

DAVID  STARR  JORDAN,  PH.  D.,  LL.  D. 

PRESIDENT   OF   LELAND    STANFORD   JUNIOR   UNIVERSITY 
AND 

VERNON  L.   KELLOGG,  M.S. 

PROFESSOR   IN    LELAND    STANFORD   JUNIOR  UNIVERSITY 


NEW    YORK 

D.    APPLETON    AND    COMPANY 
1907 


fat 


COPYRIGHT,  1900 
Br  D.   APPLETON  AND  COMPANY 


PREFACE 


THE  authors  present  this  book  as  an  elementary  ac- 
count of  animal  ecology — that  is,  of  the  relations  of  ani- 
mals to  their  surroundings  and  their  responsive  adaptation 
to  these  surroundings.  The  book  takes  the  observer's  point 
of  view,  who  is  especially  concerned  with  the  reasons  for 
the  varied  structure  and  habits  of  animals.  To  understand 
how  naturally  and  inevitably  all  animal  form,  habit,  and 
life  are  adapted  to  the  varied  circumstances  and  conditions 
of  animal  existence  should  be  the  motive  of  the  beginner  in 
this  fascinating  study.  The  greatest  facts  of  life,  except 
that  of  life  itself,  are  seen  in  the  marvelously  perfect  meth- 
ods which  Nature  has  adopted  in  the  structure  and  habits 
of  animals.  The  keen  observation  of  a  fact  should  lead 
the  student  to  inquire  into  the  significance  of  that  fact. 
The  veriest  beginner  can  be,  and  ought  to  be,  an  independ- 
ent observer  and  thinker.  In  the  study  of  zoology  that 
phase  which  treats  of  the  why  and  how  of  animal  form  and 
habit  not  only  absorbs  the  attention  of  the  most  advanced 
modern  scholars  of  biology,  but  should  also  appeal  most 
strongly  to  the  beginner.  The  beginner  and  the  most 
enlightened  thinker  in  zoology  should  each  have  the  same 
point  of  view.  With  this  belief  in  mind  the  authors  have 
tried  to  put  into  simple  form  the  principal  facts  and 
approved  hypotheses  upon  which  the  modern  conceptions 
of  animal  life  are  based. 

It  is  unnecessary  to  say  that  this  book  depends  for  its 

V 

167418 


Vl  ANIMAL  LIFE 

best  use  on  a  basis  of  personal  observational  work  by  the 
student  in  laboratory  and  field.  Without  independent 
personal  work  of  the  student  little  can  be  learned  about 
animals  and  their  life  that  will  remain  fixed.  But  present- 
day  teachers  of  biology  are  too  well  informed  to  make  a 
discussion  of  the  methods  of  their  work  necessary  here. 
As  a  matter  of  fact,  the  methods  of  the  teacher  depend  so 
absolutely  on  his  training  and  individual  initiative  that  it 
is  not  worth  while  for  the  authors  to  point  out  the  place 
of  this  book  in  elementary  zoological  teaching.  That  the 
phase  of  study  it  attempts  to  represent  should  have  a  place 
in  such  teaching  is,  of  course,  their  firm  belief. 

The  obligations  of  the  authors  for  the  use  of  certain 
illustrations  are  acknowledged  in  proper  place.  Where  no 
credit  is  otherwise  given,  the  drawings  have  been  made  by 
Miss  Mary  H.  Wellman  or  by  Mr.  James  Carter  Beard,  and 
the  photographs  have  been  made  by  the  authors  or  under 
their  direction. 

DAVID  STARR  JORDAN, 
VERNON  LYMAN  KELLOGG. 


NOTE. — After  the  pages  of  the  book  were  cast,  it  was  thought  that 
a  transposition  of  Chapters  III  and  IV  would  present  a  more  logical 
arrangement,  and  teachers  are  advised  to  omit  in  their  study  scheme 
Chapter  III  until  Chapter  IV  is  completed.  D.  S.  J. 

V.  L.  K. 


CONTENTS 


CHATTER  PAGE 

I. — THE   LIFE   OF   THE   SIMPLEST   ANIMALS 1 

The  simplest  animals,  or  Protozoa,  1.— The  animal  cell,  2. — 
What  the  primitive  cell  can  do,  5. — Amoeba,  5. — Paramoecium,  9. 
— Vorticella,  12.— Marine  Protozoa,  15.— Globigeriuffi  and  Kadio- 
laria,  16. — Antiquity  of  the  Protozoa,  20. — The  primitive  form, 
20.— The  primitive  but  successful  life,  21. 

II.— THE   LIFE   OF  THE   SLIGHTLY  COMPLEX  ANIMALS    ...        24 

Colonial  Protozoa,  24. — Gonium,  25. — Pandorina,  26. — Eudo- 
rina,  27. — Volvox,  28. — Steps  toward  complexity,  30. — Individual 
or  colony,  31. — Sponges,  32. — Polyps,  corals,  and  jelly-fishes,  37. 
— Hydra,  37. — Differentiation  of  the  body  cells,  41. — Medusje  or 
jelly-fishes,  41. — Corals,  43. — Colonial  jelly-fishes,  45. — Increase 
in  the  degree  of  complexity,  48. 

III. — THE   MULTIPLICATION   OF   ANIMALS   AND    SEX  ...         50 

All  life  from  life,  50. — Spontaneous  generation,  51. — The 
simplest  method  of  multiplication,  53. — Slightly  complex  methods 
of  multiplication,  54. — Differentiation  of  the  reproductive  cells,  55. 
— Sex,  or  male  and  female,  57. — The  object  of  sex,  57.— Sex  di- 
morphism, 58. — The  number  of  young,  61 . 

IV. — FUNCTION  AND  STRUCTURE 63 

Organs  and  functions,  63.— Differentiation  of  structure,  64.— 
Anatomy  and  physiology,  64. — The  animal  body  a  machine,  65. 
— The  specialization  of  organs,  66. — The  alimentary  canal,  66. — 
Stable  and  variable  characteristics  of  an  organ,  73. — Stable  and 
variable  characteristics  of  the  alimentary  canal,  73. — The  mutual 
relation  of  function  and  structure,  77. 

V.— THE  LIFE  CYCLE 78 

Birth,  growth  and  development,  and  death,  78. — Life  cycle  of 
simplest  animals,  78. — The  egg,  79. — Embryonic  and  post-em- 
bryonic development,  80. — Continuity  of  development,  83. — De- 
velopment after  the  gastrula  stage,  84.— Divergence  of  develop- 

vii 


Vlll  ANIMAL  LIFE 

CHAPTER  PAGE 

ment,  84. — The  laws  or  general  facts  of  development,  86. — The 
significance  of  the  facts  of  development,  89. — Metamorphosis, 
90. — Metamorphosis  among  insects,  90. — Metamorphosis  of  the 
toad,  94. — Metamorphosis  among  other  animals,  96. — Duration  of 
life,  101.— Death,  103. 

VI. — THE   PRIMARY  CONDITIONS  OF  ANIMAL   LIFE  ....      106 

Primary  conditions  and  special  conditions,  106. — Food,  106. — 
Oxygen,  107. — Temperature,  pressure,  and  other  conditions,  108. 
— Difference  between  animals  and  plants,  111. — Living  organic 
matter  and  inorganic  matter,  112. 

VII. — THE  CROWD  OF  ANIMALS  AND  THE  STRUGGLE  FOR  EXIST- 
ENCE     .  114 

The  crowd  of  animals,  114. — The  struggle  for  existence,  116. 
— Selection  by  Nature,  117. — Adjustment  to  surroundings  a  re- 
sult of  natural  selection,  120. — Artificial  selection,  120. — Depend- 
ence of  species  on  species,  121. 

VIII.— ADAPTATIONS 123 

Origin  of  adaptations,  123. — Classification  of  adaptations,  123. 
— Adaptations  for  securing  food,  125. — Adaptations  for  self-de- 
fense, 128. — Adaptations  for  rivalry,  135. — Adaptations  for  the 
defense  of  the  young,  137. — Adaptations  concerned  with  sur- 
roundings in  life,  143.— Degree  of  structural  change  in  adapta- 
tions, 146. — Vestigial  organs,  147. 

IX. — ANIMAL  COMMUNITIES  AND  SOCIAL  LIFE       .        .        .  149 

Man  not  the  only  social  animal,  149. — The  honey-bee,  149. — 
The  ants,  155.— Other  communal  insects,  158. — Gregariousness 
and  mutual  aid,  163.— Division  of  labor  and  basis  of  communal 
life,  168. — Advantages  of  communal  life,  170. 

X.— COMMENSALISM   AND   SYMBIOSIS        .          ...          *          .172 

Association  between  animals  of  different  species,  172. — Com- 
mensalism,  173. — Symbiosis,  175. 

XI. — PARASITISM  AND  DEGENERATION 179 

Relation  of  parasite  and  host,  179. — Kinds  of  parasitism,  180. 
—The  simple  structure  of  parasites,  181.— Gregarina,  182.— The 
tape-worm  and  other  flat-worms,  183.— Trichina  and  other  round- 
worms,  184.— SaccuUna,  187.— Parasitic  insects,  188. —Parasitic 
vertebrates,  193. — Degeneration  through  quiescence,  193. — De- 
generation through  other  causes,  197. — Immediate  causes  of  de- 
generation, 198. — Advantages  and  disadvantages  of  parasitism 
and  degeneration,  198. — Human  degeneration,  200. 


CONTENTS  ix 

CHAPTER  PAGE 

XII. — PROTECTIVE  RESEMBLANCES  AND  MIMICRY    ....    201 

Protective  resemblance  defined,  201. — General  protective  or 
aggressive  resemblance,  202. — Special  protective  resemblance, 
207. — Warning  colors  and  terrifying  appearances,  212. — Alluring 
coloration,  216. — Mimicry,  218. — Protective  resemblances  and 
mimicry  most  common  among  insects,  221. — No  volition  in  mim- 
icry, 222.— Color :  its  utility  and  beauty,  222. 

XIII.— THE  SPECIAL  SENSES 224 

Importance  of  the  special  senses,  224 — Difficulty  of  the  study 
of  the  special  senses,  224. — Special  senses  of  the  simplest  ani- 
mals, 225.— The  sense  of  touch,  226.— The  sense  of  taste,  228.— 
The  sense  of  smell,  229.— The  sense  of  hearing,  232.— Sound-mak- 
ing, 235  —The  sense  of  sight,  237. 

XIV. — INSTINCT  AND  REASON 240 

Irritability,  240.— Nerve  cells  and  fibers,  240.— The  brain  or 
sensorium,  241.— Reflex  action,  241.— Instinct,  242.— Classifica- 
tion of  instincts,  243.— Feeding,  244.— Self-defense,  245.— Play, 
247.— Climate,  248.— Environment,  248.— Courtship,  248.— Repro- 
duction, 249. — Care  of  the  young,  250. — Variability  of  instincts, 
251.— Reason,  251.— Mind,  255. 

XV. — HOMES  AND  DOMESTIC  HABITS 257 

Importance  of  care  of  the  young,  257.— Care  of  the  young  and 
communal  life,  257. — The  invertebrates  (except  spiders  and  in- 
sects), 258.— Spiders,  259.— Insects,  262.— The  vertebrates,  264. 

XVI. — GEOGRAPHICAL  DISTRIBUTION  OF  ANIMALS    ....    272 

Geographical  distribution,  272. — Laws  of  distribution,  274. — 
Species  debarred  by  barriers,  274. — Species  debarred  by  inability 
to  maintain  their  ground,  275. — Species  altered  by  adaptation  to 
new  conditions,  276.— Effect  of  barriers,  283.— Relation  of  species 
to  habitat,  283. — Character  of  barriers  to  distribution,  288.— Bar- 
riers affecting  fresh-water  animals,  294. — Modes  of  distribution, 
296.— Fauna  and  faunal  areas,  296.— Realms  of  animal  life,  297.— 
Subordinate  realms  or  provinces,  303.— Faunal  areas  of  the  sea, 
304. 

CLASSIFICATION  OF  ANIMALS 307 

GLOSSARY 313 

INDEX  .  .    319 


ANIMAL  LIFE 


CHAPTEE  I 

THE  LIFE   OF  THE  SIMPLEST  ANIMALS 

1.  The  simplest  animals,  or  Protozoa. — The  simplest  ani- 
mals are  those  whose  bodies  are  simplest  in  structure  and 
which  do  the  things  done  by  all  living  animals,  such  as 
eating,  breathing,  moving,  feeling,  'and  reproducing  in  the 
most  primitive  way.  The  body  of  a  horse,  made  up  of 
various  organs  and  tissues,  is  complexly  formed,  and  the 
various  organs  of  the  body  perform  the  various  kinds  of 
work  for  which  they  are  fitted  in  a  complex  way.  The 
simplest  animals  are  all  very  small,  and  almost  all  live  in 
the  water ;  some  kinds  in  fresh  water  and  many  kinds  in 
the  ocean.  Some  live  in  damp  sand  or  moss,  and  still  others 
are  parasites  in  the  bodies  of  other  animals.  They  are  not 
familiarly  known  to  us;  we  can  not  see  them  with  the 
unaided  eye,  and  yet  there  are  thousands  of  different  kinds 
of  them,  and  they  may  be  found  wherever  there  is  water. 

In  a  glass  of  water  taken  from  a  stagnant  pool  there 
is  a  host  of  animals.  There  may  be  a  few  water  beetles 
or  water  bugs  swimming  violently  about,  animals  half  an 
inch  long,  with  head  and  eyes  and  oar-like  legs ;  or  there 
may  be  a  little  fish,  or  some  tadpoles  and  wrigglers.  These 
are  evidently  not  the  simplest  animals.  There  will  be 
many  very  small  active  animals  barely  visible  to  the  un- 
aided eyes.  These,  too,  are  animals  of  considerable  com- 
plexity. But  if  a  single  drop  of  the  water  be  placed 
2  1 


2  ANIMAL  LIFE 

on  a  glass  slip  or  in  a  watch  glass  and  examined  with  a 
compound  microscope,  there  will  be  seen  a  number  of  ex- 
tremely small  creatures  which  swim  about  in  the  water-drop 
by  means  of  fine  hairs,  or  crawl  slowly  on  the  surface  of  the 
glass.  These  are  among  our  simplest  animals.  There  are, 
as  already  said,  many  kinds  of  these  "  simplest  animals," 
although,  perhaps  strictly  speaking,  only  one  kind  can  be 
called  simplest.  Some  of  these  kinds  are  spherical  in 
shape,  some  elliptical  or  football-shaped,  some  cortical,  some 
flattened.  Some  have  many  fine,  minute  hairs  projecting 
from  the  surface ;  some  have  a  few  longer,  stronger  hairs 
that  lash  back  and  forth  in  the  water,  and  some  have  no 
hairs  at  all.  There  are  many  kinds  and  they  differ  in  size, 
shape,  body  covering,  manner  of  movement,  and  habifc  of 
food-getting.  And  some  are  truly  simpler  than  others. 
But  all  agree  in  one  thing — which  is  a  very  important 
thing — and  that  is  in  being  composed  in  the  simplest  way 
possible  among  animals. 

2.  The  animal  cell — The  whole  body  of  any  one  of  the 
simplest  animals  or  Protozoa  is  composed  for  the  animal's 
whole  lifetime  of  but  a  single  cell.  The  bodies  of  all  other 
animals  are  composed  of  many  cells.  The  cell  may  be 
called  the  unit  of  animal  (or  plant)  structure.  The  body 
of  a  horse  is  complexly  composed  of  organs  and  tissues. 
Each  of  these  organs  and  tissues  is  in  turn  composed  of  a 
large  number  of  these  structural  units  called  cells.  These 
cells  are  of  great  variety  in  shape  and  size  and  general 
character.  The  cells  which  compose  muscular  tissue  are 
very  different  from  the  cells  which  compose  the  brain. 
And  both  of  these  kinds  of  cells  are  very  different  from 
the  simple  primitive,  undifferentiated  kind  of  cell  seen  in 
the  body  of  a  protozoan,  or  in  the  earliest  embryonic 
stages  of  a  many-celled  animal. 

The  animal  cell  is  rarely  typically  cellular  in  character 
— that  is,  it  is  rarely  in  the  condition  of  a  tiny  sac  or  box 
of  symmetrical  shape.  Plant  cells  are  often  of  this  char- 


THE  LIFE  OF  THE  SIMPLEST  ANIMALS  3 

acter.  The  primitive  animal  cell  (Fig.  1)  consists  of  a 
small  mass  of  a  viscid,  nearly  colorless,  substance  called 
protoplasm.  This  protoplasm  is  differentiated  to  form  two 
parts  or  regions  of  the  cell,  an  inner  denser  mass  called  the 
nucleus,  and  an  outer,  clearer,  inclosing  mass  called  the 
cytoplasm.  There  may  be  more  than 
one  nucleus  in  a  cell.  Sometimes 
the  cell  is  inclosed  by  a  cell  wall 
which  may  be  simply  a  tougher  outer 
layer  of  the  cytoplasm,  or  may  be  a 
thin  membrane  secreted  by  the  pro- 
toplasm. In  addition  to  the  proto-' 
plasm,  which  is  the  fundamental  and 
essential  cell  substance,  the  cell  may  Flo.  i._Biood  ceil  of  a  crab 
contain  certain  so-called  cell  prod-  <after  HAKCKEL).  show- 
ncta,  substances  produced  by  the  life  'S^ST^^ 

processes    Of    the    protoplasm.        The  circular  spot)  and   gran- 

cell   may  thus    contain  water,  oils, 
resin,   starch  grains,  pigment  gran- 
ules, or  other  substances.      These  substances  are   held  in 
the  protoplasm  as  liquid  drops  or  solid  particles. 

The  protoplasm  itself  of  the  cell  shows  an  obvious 
division  into  parts,  so  that  certain  parts  of  it,  especially 
parts  in  the  nucleus,  have  received  names.  The  nucleus 
usually  has  a  thin  protoplasmic  membrane  surrounding  it, 
which  is  called  the  nuclear  membrane.  There  appear  to  be 
fine  threads  or  rods  in  the  nucleus  which  are  evidently 
different  from  the  rest  of  the  nuclear  protoplasm.  These 
rods  are  called  chromosomes.  The  cell  is,  indeed,  not  so 
simple  as  the  words  "  structural  unit "  might  imply,  but 
science  has  not  yet  so  well  analyzed  its  parts  as  to  warrant 
the  transfer  of  the  name  structural  unit  to  any  single  part 
of  the  cell — that  is,  to  any  lesser  or  simpler  part  of  the 
animal  body  than  the  cell  as  a  whole. 

The  protoplasm,  which  is  the  essential  substance  of  the 
cell  and  hence  of  the  whole  animal  body,  is  a  substance 


4.  ANIMAL  LIFE 

of  a  very  complex  chemical  and  physical  constitution.  Its 
chemical  structure  is  so  complex  that  no  chemist  has  yet 
been  able  to  analyze  it,  and  as  the  further  the  attempts  at 
analysis  reach  the  more  complex  and  baffling  the  substance 
is  found  to  be,  it  is  not  improbable  that  it  may  never  be 
analyzed.  It  is  a  compound  of  numerous  substances,  some 
of  these  composing  substances  being  themselves  extremely 
complex.  The  most  important  thing  we  know  about  the 
chemical  constitution  of  protoplasm  is  that  there  are  al- 
ways present  in  it  certain  complex  albuminous  substances 
which  are  never  found  in  inorganic  bodies.  It  is  on  the 
presence  of  these  albuminous  substances  that  the  power  of 
performing  the  processes  of  life  depends.  Protoplasm  is  the 
primitive  basic  life  substance,  but  it  is  the  presence  of  these 
complex  albuminous  compounds  that  makes  protoplasm  the 
life  substance.  A  student  of  protoplasm  and  the  funda- 
mental life  processes,  Dr.  Davenport,  has  said,  "Just  as 
the  geologist  is  forced  by  the  facts  to  assume  a  vast  but 
not  infinite  time  for  earth  building,  so  the  biologist  has  to 
recognize  an  almost  unlimited  complexity  in  the  constitu- 
tion of  the  protoplasm."  * 

*  The  physical  structure  of  protoplasm  has  been  much  studied, 
but  even  with  the  improved  microscopes  and  other  instruments  neces- 
sary for  the  study  of  minute  structure,  naturalists  are  still  very  fat 
from  understanding  the  physical  constitution  of  this  substance.  While 
the  appearance  of  protoplasm  under  the  microscope  is  pretty  generally 
agreed  on  among  naturalists,  the  interpretation  of  the  kind  of  structure 
which  is  indicated  by  this  appearance  is  not  at  all  well  agreed  on. 
Protoplasm  appears  as  a  mesh  work  composed  of  fine  granules  sus- 
pended in  a  clearer  substance,  the  spaces  of  the  mesh  work  being  com- 
posed of  a  third  still  clearer  substance.  Some  naturalists  believe,  from 
this  appearance,  that  protoplasm  is  composed  of  a  clear  viscous  sub- 
tance,  in  which  are  imbedded  many  fine  granules  of  denser  substance, 
and  numerous  large  globules  of  a  clearer,  more  liquid  substance.  Other 
naturalists  believe  that  the  fine  spots  which  appear  to  be  granules  are 
simply  cross  sections  of  fine  threads  of  dense  protoplasm  which  lie 
coiled  and  tangled  in  the  thinner,  clearer  protoplasm.  And,  finally, 


THE  LIFE  OF  THE  SIMPLEST  ANIMALS  5 

3.  What  the  primitive  cell  can  do. — The  body  of  one  of 
the  minute  animals  in  the  water-drop  is  a  single  cell.     The 
body  is  not  composed  of  organs  of  different  parts,  as  in  the 
body  of  the  horse.     There  is  no  heart,  no  stomach  ;  there 
are  no  muscles,  no  nerves.     And  yet  the  protozoan  is  a  liv- 
ing animal  as  truly  as  is  the  horse,  and  it  breathes  and  eats 
and  moves  and  feels  and  produces  young  as  truly  as  does 
the  horse.     It  performs  all  the  processes  necessary  for  the 
life  of  an  animal.     The  single  cell,  the  single  minute  speck 
of  protoplasm,  has  the  power  of  doing,  in  a  very  simple  and 
primitive  way,  all  those  things  which  are  necessary  for 
life,  and  which  are  done  in  the  case  of  other  animals  by 
the  various  organs  of  the  body. 

4.  Amoeba. — The    simple   and   primitive  life  of   these 
Protozoa  can  be  best   understood  by  the   observation   of 
living  individuals.      In  the   slime  and  sediment  at  the 
bottom  of  stagnant  pools  lives  a  certain  specially  interest- 
ing kind  of  protozoan,  the  Amceba  (Fig.  2).     Of  all  the 
simplest  animals  this  is  as  simple  or  primitive  as  any.    The 
minute  viscous  particle   of  protoplasm   which   forms   its 
body  is  irregular  in  outline,  and  its  outline  or  shape  slowly 
but  constantly  changes.     It  may  contract  into  a  tiny  ball ; 
it  may  become  almost  star-shaped  ;  it  may  become  elongate 
or   flattened ;    short,  blunt,  finger-like  projections  called 
pseudopods  extend  from  the  central  body  mass,  and  these 
projections  are  constantly  changing,  slowly  pushing  out  or 

others  believe  that  protoplasm  exists  as  a  foam  work ;  that  it  is  a  vis- 
cous liquid  containing  many  fine  globules  (the  granule-appearing  spots) 
of  a  liquid  of  different  density  and  numerous  larger  globules  of  a  liquid 
of  still  other  density.  It  is  a  foam  in  which  the  bubbles  are  not  filled 
with  air,  but  with  liquids  of  different  density.  This  last  theory  of  the 
structure  of  protoplasm  is  the  one  accepted  by  a  majority  of  modern 
naturalists,  although  the  other  theories  have  numerous  believers.  But 
just  as  with  what  little  we  know  of  the  chemical  constitution  of  proto- 
plasm, the  little  we  know  of  its  physical  structure  throws  almost  no 
light  on  the  remarkable  properties  of  this  fundamental  life  substance. 


6 


ANIMAL  LIFE 


drawing  in.  The  single  protoplasmic  cell  which  makes  up 
the  body  of  the  Amoeba  has  no  fixed  outline ;  it  is  a  cell 
without  a  wall.  The  substance  of  the  cell  or  body  is  proto- 
plasm, semiliquid  and  colorless.  The  changes  in  form  of 
the  body  are  the  moving  of  the  Amoeba.  By  close  watching 
it  may  be  seen  that  the  Amoeba  changes  its  position  on  the 
glass  slip.  Although  provided  with  no  legs  or  wings  or 


FIG.  2. — An  Amoeba,  showing  different  shapes  assumed  by  it  when  crawling. 
—After  VEBWOBN. 

scales  or  hooks — that  is,  with  no  special  organs  of  locomo- 
tion— the  Amoeba  moves.  There  are  no  muscles  in  this  tiny 
body;  muscles  are  composed  of  many  contractile  cells 
massed  together,  and  the  Amoeba  is  but  one  cell.  But  it  is 
a  contractile  cell ;  it  can  do  what  the  muscles  of  the  com- 
plex animals  do. 

If  one  of  the  finger-like  projections  of  the  Amoeba^  or, 
indeed,  if  any  part  of  its  body  comes  in  contact  with  some 
other  microscopic  animal  or  plant  or  some  small  fragment 
of  a  larger  form,  the  soft  body  of  the  Amoeba  will  be  seen 


THE  LIFE  OF  THE  SIMPLEST  ANIMALS 


to  press  against  it,  and  soon  the  plant  or  animal  or  organic 
particle  becomes  sunken  in  the  protoplasm  of  the  formless 
body  and  entirely  inclosed  in  it  (Fig.  3).  The  absorbed 
particle  soon  wholly  or  partly  disappears.  This  is  the 
manner  in  which  the  Amoeba  eats.  It  has  no  mouth  or 


FIG.  3.— Am&ba  eating  a  microscopic  one-celled  plant.— After  VERWOKN. 

stomach.  Any  part  of  its  body  mass  can  take  in  and  digest 
food.  The  viscous,  membraneless  body  simply  flows  about 
the  food  and  absorbs  it.  Such  of  the  food  particles  as  can 
not  be  digested  are  thrust  out  of  the  body. 

The  Amoeba  breathes.  Though  we  can  not  readily  ob- 
serve this  act  of  respiration,  it  is  true  that  the  Amoeba  takes 
into  its  body  through  any  part  of  its  surface  oxygen  from 
the  air  which  is  mixed  with  water,  and  it  gives  off  from  any 
part  of  its  body  carbonic-acid  gas.  Although  the  Amoeba 
has  no  lungs  or  gills  or  other  special  organs  of  respiration, 
it  breathes  in  oxygen  and  gives  out  carbonic-acid  gas,  which 
is  just  what  the  horse  does  with  its  elaborately  developed 
organs  of  respiration. 

If  the  Amoeba,  in  moving  slowly  about,  comes  into  con- 
tact with  a  sand  grain  or  other  foreign  particle  not  suitable 
for  food,  the  soft  body  slowly  recoils  and  flows — for  the 
movement  is  really  a  flowing  of  the  thickly  fluid  protoplasm 
— so  as  to  leave  the  sand  grain  at  one  side.  The  Amoeba 
feels.  It  shows  the  effects  of  stimulation.  Its  movements 
can  be  changed,  stopped,  or  induced  by  mechanical  or 
chemical  stimuli  or  by  changes  in  temperature.  The 


8 


ANIMAL  LIFE 


Amoeba  is  irritable ;  it  possesses  irritability,  which  is  sensa- 
tion in  its  simplest  degree. 

If  food  is  abundant  the  Amoeba  soon  increases  in  size. 
The  bulk  of  its  body  is  bound  to  increase  if  new  substance 


FIG.  4.— Amoeba  polypodia  in  six  successive  stages  of  division.     The  dark,  white- 
margined  spot  in  the  interior  is  the  nucleus.— After  P.  E.  SCHULZE. 

is  constantly  assimilated  and  added  to  it.  The  Amoeba 
grows.  But  there  seem  to  be  some  fixed  limits  to  the 
extent  of  this  increase  in  size.  No  Amoeba  becomes  large. 
A  remarkable  phenomenon  always  oocurs  to  prevent  this, 


THE  LIFE  OF  THE  SIMPLEST  ANIMALS  9 

An  Amoeba  which  has  grown  for  some  time  contracts  all 
its  finger-like  processes,  and  its  body  becomes  constricted. 
This  constriction  or  fissure  increases  inward,  so  that  the 
body  is  soon  divided  fairly  in  two  (Fig.  4).  The  body, 
being  an  animal  cell,  possesses  a  nucleus  imbedded  in  the 
body  protoplasm  or  cytoplasm.  When  the  body  begins  to 
divide,  the  nucleus  begins  to  divide  also,  and  becomes  en- 
tirely divided  before  the  fission  of  the  cytoplasm  is  com- 
plete. There  are  now  two  Amcebce,  each  half  the  size  of 
the  original  one  ;  each,  indeed,  being  actually  one  half  of 
the  original  one.  This  splitting  of  the  body  of  the  Amoeba, 
which  is  called  fission,  is  the  process  of  reproduction.  The 
original  Amoeba  is  the  parent ;  the  two  halves  of  the  parent 
are  the  young.  Each  of  the  young  possesses  all  of  the 
characteristics  and  powers  of  the  parent ;  each  can  move, 
eat,  feel,  grow,  and  reproduce  by  fission.  It  is  very  evident 
that  this  is  so,  for  any  part  of  the  body  or  the  whole  body 
was  used  in  performing  these  functions,  and  the  young  are 
simply  two  parts  of  the  parent's  body.  But  if  there  be  any 
doubt  about  the  matter,  observation  of  the  behavior  of  the 
young  or  new  Amcebce  will  soon  remove  it.  Each  puts  out 
pseudopods,  moves,  ingests  food  particles,  avoids  sand 
grains,  contracts  if  the  water  is  heated,  grows,  and  finally 
divides  in  two. 

5.  Paramcecimn. — Another  protozoan  which  is  common 
in  stagnant  pools  and  can  be  readily  obtained  and  observed 
is  Paramcecium  (Fig.  5).  The  body  of  the  Paramcecium  is 
much  larger  than  that  of  the  Amoeba,  being  nearly  one  fourth 
of  a  millimeter  in  length,  and  is  of  fixed  shape.  It  is  elon- 
gate, elliptical,  and  flattened,  and  when  examined  under  the 
microscope  seems  to  be  a  very  complexly  formed  little  mass. 
The  body  of  the  Paramcecium  is  indeed  less  primitive  than 
that  of  the  Amoeba,  and  yet  it  is  still  but  a  single  cell. 
The  protoplasm  of  the  body  is  very  soft  within  and  dense 
on  the  outside,  and  it  is  covered  externally  by  a  thin  mem- 
brane. The  body  is  covered  with  short  fine  hairs  or  cilia, 


10 


ANIMAL  LIFE 


which  are  fine  processes  of  the  dense  protoplasm  of  the 
surface.  There  is  on  one  side  an  oblique  shallow  groove 
that  leads  to  a  small,  funnel-shaped  depression  in  the  body 
which  serves  as  a  primitive  sort  of  mouth 
or  opening  for  the  ingress  of  food. 
The  ParamcBcium  swims  about  in  the 
water  by  vibrating  the  cilia  which  cov- 
er the  body,  and  brings  food  to  the 
mouth  opening  by  producing  tiny  cur- 
rents in  the  water  by  means  of  the 
cilia  in  the  oblique  groove.  The  food, 
which  consists  of  other  living  Proto- 
zoa, is  taken  into  the  body  mass  only 
through  the  funnel-shaped  opening,  and 
that  part  of  it  which  is  undigested  is 
thrust  out  always  through  a  particular 
part  of  the  body  surface.  (The  taking 
in  and  ejecting  of  foreign  particles  can 
be  seen  by  putting  a  little  powdered 
carmine  in  the  water.)  Within  the 
body  there  are  two  nuclei  and  two  so- 
called  pulsating  vacuoles.  These  pul- 
sating vacuoles  (Amceba  has  one)  seem 
to  aid  in  discharging  waste  products 
When  the  Paramce- 
cium  touches  some  foreign  substance  or 
is  otherwise  irritated  it  swims  away, 
and  it  shoots  out  from  the  surface  of  its  body  some  fine 
long  threads  which  when  at  rest  are  probably  coiled  up  in 
little  sacs  on  the  surface  of  the  body.  When  the  Parar 
mcecium  has  taken  in  enough  food  and  grown  so  that  it 
has  reached  the  limit  of  its  size,  it  divides  transversely  into 
halves  as  the  Amoeba  does.  Both  nuclei  divide  first,  and 
then  the  cytoplasm  constricts  and  divides  (Fig.  6).  Thus 
two  new  ParamoBcia  are  formed.  One  of  them  has  to  de- 
velop a  new  mouth  opening  and  groove,  so  that  there  is  in 


FIG.  S.—Paramaicium  au- 
relia  (after  VKRWOKN). 
At  each  end  there  is  a 
contractile  vacuole,  and  from  the  body, 
in  the  center  is  one  of 
the  nuclei. 


THE  LIFE  OF  THE  SIMPLEST  ANIMALS 


11 


the  case  of  the  reproduction  of  Paramcecium  the  beginnings 
of  developmental  changes  during  the  course  of  the  growth 
of  the  young.  The  young  Amcebce  have  only  to  add  sub- 
stance to  their  bodies,  to  grow  larger,  in  order  to  be  exactly 
like  their  parent. 

The  new  Paramcecia  attain  full  size  and  then  divide, 
each  into  two.  And  so  on  for  many  generations.  But  it 
has  been  discovered  that  this  simplest  kind  of  reproduction 
can  not  go  on  indefinitely.  After  a  number  of  generations 
the  Paramcecia,  instead  of  simply  dividing  in  two,  come 

together  in  pairs,  and  a  part  of 
one  of  the  nuclei  of  each  mem- 
ber of  a  pair  passes  into  the 
body  of  and  fuses  with  a  part 


FIG.  6. — Paramcecium  putorinum 
dividing.  The  two  nuclei  be- 
come very  elongate  before  di- 
viding.— After  BiJTscHLi. 


PIG.  7. — Paramcecium  caudatum  ,*  two  indi- 
viduals separating  after  conjugation. 


of  one  of  the  nuclei  of  the  other  member  of  the  pair.  In 
the  meantime  the  second  nucleus  in  each  Paramcecium  has 
broken  up  into  small  pieces  and  disappeared.  The  new 
nucleus  composed  of  parts  of  the  nuclei  from  two  animals 
divides,  giving  each  animal  two  nuclei  just  as  it  had  before 
this  extraordinary  process,  which  is  called  conjugation, 
began  (Fig.  7).  Each  Paramcecium,  with  its  nuclei  com- 
posed of  parts  of  the  nuclei  from  two  distinct  individuals, 


12 


ANIMAL  LIFE 


now  simply  divides  in  two,  and  a  large  number  of  genera- 
tions by  simple  fission  follow. 

Paramcecium  in  the  character  of  its  body  and  in  the 
manner  of  the  performance  of  its  life  processes  is  distinctly 
less  simple  than  the  Amoeba,  but  its  body  is  composed  of  a 
single  structural  unit,  a  single  cell,  and  it  is  truly  one  of 
the  "  simplest  animals." 

6.  Vorticella, — Another  interesting  and  readily  found 
protozoan  is  Vorticella  (Fig.  8).  While  the  Amoeba  can  crawl 
and  Paramcecium  swim,  Vorticella,  except  when  very  young, 


Pie.  8— Vorticella  microstoma  (after  STEIN).  A,  small,  free-swimming  individuals 
conjugating  with  a  large,  stalked  individual ;  B,  a  stalked  individual  dividing 
longitudinally ;  C,  after  division  is  completed  one  part  severs  itself  from  the 
other,  forms  a  ring  of  cilia,  and  swimfl  away. 

is  attached  by  tiny  stems  to  dead  leaves  or  sticks  in  the 
water,  and  can  change  its  position  only  to  a  limited  extent. 


THE  LIFE  OF  THE  SIMPLEST  ANIMALS  13 

The  body  is  pear-shaped  or  bell-shaped,  with  a  mouth 
opening  at  the  broad  end,  and  a  delicate  stem  at  the 
narrow  end.  This  stem  is  either  hard  and  stiff,  or  is 
flexible  and  capable  of  being  suddenly  contracted  in  a 
close  spiral.  In  the  body  mass  there  is  one  pulsating 
vacuole  and  one  nucleus.  Usually  many  Vorticellce  are 
found  together  on  a  common  stalk,  thus  forming  a  proto- 
zoan colony. 

The  life  processes  of  Vorticella  are  of  the  simple  kind 
already  observed  in  Amoeba  and  Paramoecium.  Vorticella 
shows,  however,  some  modifications  of  the  process  of  repro- 
duction which  are  interesting.  The  plane  of  division  of 
Vorticella  is  parallel  to  the  long  axis  of  the  pear-shaped 
body,  so  that  when  fission  is  complete  there  are  two  Vorti- 
cellce on  a  single  stalk.  One  of  the  two  becomes  detached, 
and  by  means  of  a  circle  of  fine  hairs  or  cilia  which  appear 
around  its  basal  end  leads  a  free  swimming  life  for  a  short 
time.  Finally  it  settles  down  and  develops  a  stalk.  Vorti- 
cella shows  two  kinds  of  fission — one  the  usual  division 
into  equal  parts,  and  another  division  into  unequal  parts. 
In  this  latter  kind,  called  reproduction  or  multiplication 
by  budding,  a  small  part  of  the  parent  body  separates, 
develops  a  basal  circle  of  cilia,  and  swims  away.  The  pro- 
cess of  conjugation  also  takes  place  among  the  Vorti- 
cella^ but  they  are  never  two  equal  forms  which  conju- 
gate, but  always  one  of  the  ordinary  stalked  forms  and 
one  of  the  small  free  -  swimming  forms  produced  by 
budding. 

Here,  then,  in  the  life  of  Vorticella,  are  new  modifica- 
tions of  the  life  processes  ;  but,  after  all,  these  life  processes 
are  very  simply  performed,  and  the  body  is  like  the  body  of 
the  Amceba,  a  single  cell.  Vorticella  is  plainly  one  of  "  the 
simplest  animals." 

7.  Gregarina. — A  fourth  kind  of  protozoan  to  which  we 
can  profitably  give  some  special  attention  is  Gregarina 
(Fig.  9),  the  various  species  of  which  live  in  the  alimentary 


ANIMAL  LIFE 


canal*  of  crayfishes  and  centipeds  and  certain  insects. 
Gregarina  is  a  parasite,  living  at  the  expense  of  the  host 
in  whose  body  it  lies.  It  has  no  need  to  swim  about  quickly, 


FIG.  9.— Gregarinidae.  A,  a  Gregarinid  (Actinocephalus  oligacanthus)  from  the  intes- 
tine of  an  insect  (after  STEIN)  ;  B  and  C,  spore  forming  by  a  Gregarinid  (Coc- 
cidium  oviforme)  from  the  liver  of  a  guinea-pig  (after  LEUCKART)  ;  D,  E,  and 
P,  successive  stages  in  the  conjugation  and  spore  forming  of  Gregarina  poly- 
morpha  (after  KOBLLIKER). 

and  hence  has  no  swimming  cilia  like  Paramcecium  and 
the  young  Vorticella.  It  does  need  to  cling  to  the  inner 
wall  of  the  alimentary  canal  of  its  host,  and  the  body  of 
some  species  is  provided  with  hooks  for  that  purpose.  The 

*  Specimens  of  Oregarina  can  be  abundantly  found  in  the  alimen- 
tary canal  of  meal  worms,  the  larvae  of  the  black  beetle  (Tenebrio  moli- 
tor),  common  in  granaries,  mills,  and  brans.  "Snip  off  with  small 
scissors  both  ends  of  a  larva,  seize  the  protruding  (white)  intestine  with 
forceps,  draw  it  out,  and  tease  a  portion  in  normal  salt  solution  (water 
will  do)  on  a  slide.  Cover,  find  with  the  low  power  (minute,  oblong, 
transparent  bodies),  and  study  with  any  higher  objective  to  suit."— 

MURBACH. 


THE  LIFE  OF   THE  SIMPLEST  ANIMALS  15 

food  of  Gregarina  is  the  liquid  food  of  the  host  as  it  exists 
in  the  intestine,  and  which  is  simply  absorbed  anywhere 
through  the  surface  of  the  body  of  the  parasite.  There  is 
no  mouth  opening  nor  fixed  point  of  ejection  of  waste 
material,  nor  is  there  any  contractile  vacuole  in  the  body. 

In  the  method  of  multiplication  or  reproduction  Gre- 
garina shows  an  interesting  difference  from  Amceba  and 
Paramcecium  and  Vorticella.  When  the  Gregarina  is 
ready  to  multiply,  its  body,  which  in  most  species  is  rather 
elongate  and  flattened,  contracts  into  a  ball-shaped  mass 
and  becomes  encysted — that  is,  becomes  inclosed  in  a  tough, 
membranous  coat.  This  may  in  turn  be  covered  externally 
by  a  jelly-like  substance.  The  nucleus  and  the  protoplasm 
of  the  body  inside  of  the  coat  now  divide  into  many  small 
parts  called  spores,  each  spore  consisting  of  a  bit  of  the 
cytoplasm  inclosing  a  small  part  of  the  original  nucleus. 
Later  the  tough  outer  wall  of  the  cyst  breaks  and  the 
spores  fall  out,  each  to  grow  and  develop  into  a  new  Gre- 
garina. In  some  species  there  are  fine  ducts  or  canals 
leading  from  the  center  of  the  cyst  through  the  wall  to  the 
outside,  and  through  these  canals  the  spores  issue.  Some- 
times two  GregarincB  come  together  before  encystation  and 
become  inclosed  in  a  common  wall,  the  two  thus  forming  a 
single  cyst.  This  is  a  kind  of  conjugation.  In  some  spe- 
cies each  of  the  young  or  new  Gregarince  coming  from  the 
spores  immediately  divides  by  fission  to  form  two  indi- 
viduals. 

8.  Marine  Protozoa. — If  called  upon  to  name  the  char- 
acteristic animals  of  the  ocean,  we  answer  readily  with  the 
names  of  the  better-known  ocean  fishes,  like  the  herring  and 
cod,  which  we  know  to  live  there  in  enormous  numbers ;  the 
seals  and  sea  lions,  the  whales  and  porpoises,  those  fish-like 
animals  which  are  really  more  like  land  animals  than  like 
the  true  fishes ;  and  the  jelly-fishes  and  corals  and  star-fishes 
which  abound  along  the  ocean's  edge.  But  in  naming  only 
these  we  should  be  omitting  certain  animals  which  in  point 


16  ANIMAL  LIFE 

of  abundance  of  individuals  vastly  outnumber  all  other 
animals,  and  which  in  point  of  importance  in  helping  main- 
tain the  complex  and  varied  life  of  the  ocean  distinctly  out- 
class all  other  marine  forms.  These  animals  are  the  marine 
Protozoa,  those  of  the  "  simplest  animals  "  which  live  in  the 
ocean. 

Although  the  water  at  the  surface  of  the  ocean  appears 
clear,  and  on  superficial  examination  devoid  of  life,  yet  a 
drop  of  this  water  taken  from  certain  ocean  regions  exam- 
ined under  the  microscope  reveals  the  fact  that  this  water 
is  inhabited  by  Protozoa.  Not  only  is  the  water  at  the 
very  surface  of  the  ocean  the  home  of  the  simplest  animals, 
but  they  can  be  found  in  all  the  water  from  the  surface  to 
a  great  depth  beneath  it.  In  a  pint  of  this  ocean  water 
from  the  surface  or  near  it  there  may  be  millions  of  these 
animals.  In  the  oceans  of  the  world  the  number  of  them 
is  inconceivable.  Dr.  W.  K.  Brooks  says  that  the  "  basis 
of  all  the  life  in  the  modern  ocean  is  found  in  the  micro- 
organisms of  the  surface."  By  micro-organisms  he  moans 
the  one-celled  animals  and  the  one-celled  plants.  For 
the  simplest  plants  are,  like  the  simplest  animals,  one- 
celled.  "  Modern  microscopical  research,"  he  says,  "  has 
shown  that  these  simple  plants,  and  the  Globigerinae  and 
Eadiolaria  [kinds  of  Protozoa]  which  feed  upon  them,  are 
so  abundant  and  prolific  that  they  meet  all  demands  and 
supply  the  food  for  all  the  animals  of  the  ocean." 

9.  The  Globigerinse  and  Radiolaria.— The  Globigerinse 
(Fig.  10)  and  Radiolaria  (Fig.  11)  are  among  the  most  in- 
teresting of  all  the  simplest  animals.  Their  simple  one- 
celled  body  is  surrounded  by  a  microscopic  shell,  which 
among  the  Globigerinae  is  usually  made  of  lime  (calcium 
carbonate),  in  the  case  of  Eadiolaria  of  silica.  These  minute 
shells  present  a  great  variety  of  shape  and  pattern,  many 
being  of  the  most  exquisite  symmetry  and  beauty.  The 
shells  are  usually  perforated  by  many  small  holes,  through 
which  project  long,  delicate,  protoplasmic  threads.  These 


THE  LIFE  OF  THE  SIMPLEST  ANIMALS 


17 


fine  threads  interlace  when  they  touch  each  other,  thus 
forming  a  sort  of  protoplasmic  network  outside  of  the  shell. 
In  some  cases  there  is  a  complete  layer  of  protoplasm — 
part  of  the  body  protoplasm  of  the  protozoan  — surround- 


FIG.  lO.—Polystomella  strigillata,  one  of  the  Globigerinae.  —  After  MAX  SCHTTLTZE. 

ing  the  cell  externally.  The  Kadiolaria,  whose  shells  are 
made  of  silica,  possess  also  a  perforated  membranous  sac 
called  the  central  capsule,  which  lies  imbedded  in  the 
protoplasm,  dividing  it  into  two  portions,  one  within  and 
3 


18  ANIMAL  LIFE  . 

one  outside  of  the  capsule.  In  the  protoplasm  inside  of 
the  capsule  lies  the  nucleus  or  nuclei ;  and  from  the  proto- 
plasm outside  of  the  capsule  rise  the  numerous  fine,  thread- 
like pseudopods  which  project  through  the  apertures  in  the 
shell,  and  enable  the  animal  to  swim  and  to  get  food. 

Most  of  the  myriads  of  the  simplest  animals  which 
swarm  in  the  surface  waters  of  the  ocean  belong  to  a  few 
kinds  of  these  shell-bearing  Globigerinae  and  Radiolaria. 
Large  areas  of  the  bottom  of  the  Atlantic  Ocean  are  cov- 
ered with  a  slimy  gray  mud,  often  of  great  thickness,  which 
is  called  globigerina-ooze,  because  it  is  made  up  chiefly  of 
the  microscopic  shells  of  Globigerinae.  As  death  comes  to 
the  minute  protoplasmic  animals  their  hard  shells  sink 
slowly  to  the  bottom,  and  accumulate  in  such  vast  quanti- 
ties as  to  form  a  thick  layer  on  the  ocean  floor.  Nor  is  it 
only  in  present  times  and  in  the  oceans  we  know  that  the 
Globigerinae  have  flourished.  All  over  the  world  there  are 
thick  rock  strata  which  are  composed  chiefly  of  the  fos- 
silized shells  of  these  simplest  animals.  Where  the  strata 
are  made  up  exclusively  of  these  shells  the  rock  is  chalk. 
Thus  are  composed  the  great  chalk  cliffs  of  Kent,  which 
gave  to  England  the  early  name  of  Albion,  and  the  chalk 
beds  of  France  and  Spain  and  Greece.  The  existence  of 
these  chalk  strata  means  that  where  now  is  land,  in  earlier 
geologic  times  were  oceans,  and  that  in  the  oceans  Globi- 
gerinae lived  in  countless  numbers.  Dying,  their  shells 
accumulated  to  form  thick  layers  on  the  sea  bottom.  In 
later  geologic  ages  this  sea  bottom  has  been  uplifted  and 
is  now  land,  far  perhaps  from  any  ocean.  The  chalk  strata 
of  the  plains  of  the  United  States,  like  those  in  Kansas,  are 
more  than  a  thousand  miles  from  the  sea,  and  yet  they  are 
mainly  composed  of  the  fossilized  shells  of  marine  Pro- 
tozoa. Indeed,  we  are  acquainted  with  more  than  twice  as 
many  fossil  species  of  Globigerinae  as  species  living  at  the 
present  time.  The  ancestors  of  these  Globigerinae,  from 
which  the  present  Globigerinae  differ  but  little,  can  be 


THE  LIFE  OF  THE  SIMPLEST  ANIMALS 


19 


traced  far  back  in  the  geologic  history  of  the  world.     It  is 
an  ancient  type  of  animal  structure. 

The  Radiolaria,  too,  which  live  abundantly  in  the  pres- 
ent oceans,  especially  in  the  marine  waters  of  the  tropical 
and  temperate  zones,  are  found  as  fossils  in  the  rocks  from 
the  time  of  the  coal  age  on.  The  siliceous  shells  of  the 


FIG.  ll.—Heliosphcera  actinota  (after  HAECKEL)  ;    a  radiolarian  with  symmetrical 

shell. 

Eadiolaria  sinking  to  the  sea  bottom  and  accumulating 
there  in  great  masses  form  a  radiolaria-ooze  similar  to  the 
globigerinae-ooze ;  and  just  as  with  the  Globigerinae,  the 
remains  of  the  ancient  Radiolaria  formed  thick  layers  on 
the  floor  of  the  ancient  oceans,  which  have  since  been  up- 
lifted and  now  form  certain  rock  strata.  That  kind  of 
rock  called  Tripoli,  found  in  Sicily,  and  the  Barbados 
earth  from  the  island  of  Barbados,  both  of  which  are  used 


20  ANIMAL  LIFE 

as  polishing  powder,  are  composed  almost  exclusively  of 
the  siliceous  shells  of  ancient  and  long-extinct  Radiolaria. 

10.  Antiquity  of  the  Protozoa, — All  the  animals  of  the 
ocean  depend  upon  the  marine  Protozoa  (and  the  marine 
Protophyta,  or  one-celled  plants)  for  food.     Either  they 
prey  upon  these  one-celled  organisms  directly,  or  they  prey 
upon  animals  which  do  prey  on  these  simplest  animals. 
The  great  zoologist  already  quoted  says :  "  The  food  sup- 
ply of  marine  animals  consists  of  a  few  species  of  micro- 
scopic   organisms  which  are  inexhaustible  and  the  only 
source  of  food  for  all  the  inhabitants  of  the  ocean.     The 
supply  is  primeval  as  well  as  inexhaustible,  and  all  the  life 
of  the  ocean  has  gradually  taken  shape  in  direct  depend- 
ence upon  it."    That  is,  the  marine  simplest  animals  are 
the  only  marine  animals  which  live  independently;  they 
alone  can  live  or  could  have  lived  in  earlier  ages  without 
depending  on  other  animals.     They  must  therefore  be  the 
oldest  of  marine  animals.     By  oldest  we  mean  that  their 
kind  appeared  earliest  in  the  history  of  the  world.     As  it 
is  certain  that  marine  life  is  older  than  terrestrial  life — that 
is,  that  the  first  animals  lived  in  the  ocean — it  is  obvious 
that  the  marine  Protozoa  are  the  most  ancient  of  animals. 
This  is  an  important  and  interesting  fact.     Zoologists  try 
to  find  out  the  relationships  and  the  degrees  of  antiquity 
or  modernness  of  the  various  kinds  of  animals.     We  have 
seen  that  the  Protozoa,  those  animals  which  have  the  sim- 
plest body  structure  and  perform  the  necessary  life  pro- 
cesses in  the  simplest  way,  are  the  oldest,  the  first  animals. 
Tlrs  is  just  what  we  would  expect. 

11.  The  primitive  form. — We  find  among  the  simplest 
animals  a  considerable  variety  of  shape  and  some  manifest 
variation  in  habit.     But  the  points  of  resemblance  are  far 
more  pronounced  than  the  points  of  difference,  and  are  of 
fundamental  importance.     The  composition  of  the  body  of 
one  cell,  as  opposed  to  the  many-celled  structure  of  the 
bodies  of  all  other  animals,  is  the  fact  to  be  most  distinctly 


THE  LIFE  OF  THE  SIMPLEST  ANIMALS  21 

emphasized.  The  shape  of  this  one-celled  body  varies. 
With  the  most  primitive  or  simplest  of  the  "  simplest  ani- 
mals," like  Amceba,  for  example,  there  is  no  "  distinction 
of  ends,  sides,  or  surfaces,  such  as  we  are  familiar  with  in 
in  the  higher  animals.  Anterior  and  posterior  ends,  right 
and  left  sides,  dorsal  and  ventral  surfaces  are  terms  which 
have  no  meaning  in  reference  to  an  Amoeba,  for  any  part 
of  the  animal  may  go  first  in  locomotion,  and  when  crawl- 
ing the  animal  moves  along  on  whatever  part  of  its 
surface  happens  to  be  in  contact  with  foreign  bodies." 
The  one  shape  most  often  seen  among  the  Protozoa,  or 
most  nearly  fairly  to  be  called  the  typical  shape,  is  the 
spherical  or  subspherical  shape.  Why  this  is  so  is  readily 
seen.  Most  of  the  Protozoa  are  aquatic  and  free  swim- 
ming. They  live  in  a  medium,  the  water,  which  supports 
or  presses  on  the  body  equally  on  all  sides,  and  the  body  is 
not  forced  to  assume  any  particular  form  by  the  environ- 
ment. The  body  rests  suspended  in  the  water  with  any 
part  of  its  surface  uppermost  or  any  part  undermost.  As 
any  part  of  the  surface  serves  equally  well  in  many  of  the 
Protozoa  for  breathing  or  eating  or  excreting,  it  is  obvious 
that  the  spherical  form  is  the  simplest  and  most  conven- 
ient shape  for  such  a  body.  It  is  interesting  to  note  that 
the  spherical  form  is  the  common  shape  of  the  egg  cell  of 
the  higher  animals.  Each  one  of  the  higher,  multicellular 
animals  begins  life  (as  we  shall  find  it  explained  in  another 
chapter  of  this  book)  as  a  single  cell,  the  egg  cell,  and 
these  egg  cells  are  usually  spherical  in  shape.  The  full 
significance  of  this  we  need  not  now  attempt  to  under- 
stand, but  it  is  interesting  to  note  that  normally  the  whole 
body  of  the  simplest  animals  is  a  single  spherical  cell,  and 
that  every  one  of  the  higher  animals,  however  complex 
it  may  become  by  growth  and  development,  begins  life  as  a 
single  spherical  cell. 

12.  The  primitive  but  successful  life. — Living  consists  of 
the  performing  of  certain  so-called  life  processes,  such  as 


22  ANIMAL  LIFE 

eating,  breathing,  feeling,  and  multiplying.  These  pro- 
cesses are  performed  among  the  higher  animals  by  various 
organs,  special  parts  of  the  body,  each  of  which  is  fitted  to 
do  some  one  kind  of  work,  to  perform  some  one  of  these 
processes.  There  is  a  division  or  assignment  of  labor  here 
among  different  parts  of  the  body.  Such  a  division  of 
labor,  and  special  fitting  of  different  parts  of  the  body  for 
special  kinds  of  work  does  not  exist,  or  exists  only  in 
slightest  degree  among  the  simplest  animals.  The  Amceba 
eats  or  feels  or  moves  with  any  part  of  its  body ;  all  of  the 
body  exposed  to  the  air  (air  held  in  the  water)  breathes ; 
the  whole  body  mass  takes  part  in  the  process  of  repro- 
duction. 

Only  very  small  organisms  can  live  in  this  simplest  way. 
So  all  of  the  Protozoa  are  minute.  When  the  only  part  of 
the  body  which  can  absorb  oxygen  is  the  simple  external 
surface  of  a  spherical  body,  the  mass  of  that  body  must  be 
very  small.  With  any  increase  in  size  of  the  animal  the 
mass  of  the  body  increases  as  the  cube  of  the  diameter, 
while  the  surface  increases  only  as  the  square  of  the  diam- 
eter. Therefore  the  part  of  the  body  (inside)  which  re- 
quires to  be  provided  with  oxygen  increases  more  rapidly 
than  the  part  (the  outside)  which  absorbs  oxygen.  Thus 
this  need  of  oxygen  alone  is  sufficient  to  determine  the 
limit  of  size  which  can  be  attained  by  the  spherical  or  sub- 
spherical  Protozoa. 

That  the  simplest  animals,  despite  the  lack  of  organs 
and  the  primitive  way  of  performing  the  life  processes,  live 
successfully  is  evident  from  their  existence  in  such  ex- 
traordinary numbers.  They  outnumber  all  other  animals. 
Although  serving  as  food  for  hosts  of  ocean  animals,  the 
marine  Protozoa  are  the  most  abundant  in  individuals  of 
all  living  animals.  The  conditions  of  life  in  the  surface 
waters  of  the  ocean  are  easy,  and  a  simple  structure  and 
simple  method  of  performance  of  the  life  processes  are 
wholly  adequate  for  successful  life  under  these  conditions. 


THE  LIFE  OF  THE  SIMPLEST  ANIMALS  23 

That  the  character  of  the  body  structure  of  the  Protozoa 
has  changed  but  little  since  early  geologic  times  is  ex- 
plained by  the  even,  unchanging  character  of  their  sur- 
roundings. The  oceans  of  former  ages  have  undoubtedly 
been  essentially  like  the  oceans  of  to-day — not  in  extent 
and  position,  but  in  their  character  of  place  of  habitation 
for  animals.  The  environment  is  so  simple  and  uniform 
that  there  is  little  demand  for  diversity  of  habits  and  conse- 
quent diversity  of  body  structure.  Where  life  is  easy  there 
is  no  necessity  for  complex  structure  or  complicated  habits 
of  living.  So  the  simplest  animals,  unseen  by  us,  and  so 
inferior  to  us  in  elaborateness  of  body  structure  and  habit, 
swarm  in  countless  hordes  in  all  the  oceans  and  rivers  and 
lakes,  and  live  successfully  their  simple  lives. 


CHAPTER  II 

THE  LIFE  OF  THE  SLIGHTLY  COMPLEX  ANIMALS 

13.  Colonial  Protozoa,— When  one  of  the  simplest  animals 
multiplies  by  fission,  the  halves  of  the  one-celled  body  sepa- 
rate wholly  from  each  other,  move  apart,  and  pursue  their 
lives  independently.  The  original  parent  cell  divides  to 
form  two  cells,  which  exist  thereafter  wholly  apart  from 
each  other.  There  are,  however,  certain  simple  animals 
which  are  classed  with  the  Protozoa,  which  show  an  inter- 
esting and  important  difference  from  the  great  majority  of 
the  simplest  animals.  These  are  the  so-called  colony-form- 
ing or  colonial  Protozoa. 

These  colonial  Protozoa  belong  to  a  group  of  organisms 
called  the  *  Volvocinae.  The  simplest  of  the  Volvocinae  are 
single  cells,  which  live  wholly  independently  and  are  in 
structure  and  habit  essentially  like  the  other  Protozoa  we 
have  studied.  They  have,  however,  imbedded  in  the  one- 
celled  body  a  bit  of  chlorophyll,  the  green  substance  which 
gives  the  color  to  green  plants  and  is  so  important  in  their 
physiology.  In  this  respect  they  differ  from  the  other 
Protozoa.  Among  the  other  Volvocinae,  however,  a  few  or 
many  cells  live  together,  forming  a  small  colony — that  is, 

*  These  colonial  organisms,  the  Volvocinae,  are  the  objects  of  some 
contention  between  botanists  and  zoologists.  The  botanists  call  them 
plants  because  they  possess  a  cellulose  membrane  and  green  chroma- 
tophores,  and  exhibit  the  metabolism  characteristic  of  most  plants  ;  but 
most  zoologists  consider  them  to  be  animals  belonging  to  the  order 
Flagellata  of  the  Protozoa.  In  the  latest  authoritative  text-book  of 
zoology,  that  of  Parker  and  Haswell  (1897),  they  are  so  classed. 
34 


THE  LIFE  OF  THE  SLIGHTLY  COMPLEX  ANIMALS       25 

there  is  formed  a  group  of  a  few  or  many  cells,  each  cell 
having  the  structure  of  the  simpler  unicellular  forms. 
These  cells  are  held  together  in  a  gelatinous  envelope,  and 
the  mass  is  usually  spherical  in  shape.  In  most  of  the 
colonies  each  of  the  cells  possesses  two  or  three  long,  pro- 
toplasmic, whiplash-like  hairs,  called  flagella,  and  by  the 
lashing  of  these  flagella  in  the  water  the  whole  group  swims 
about. 

14.  Gonium. — If,  when  one  of  the  simplest  animals  di- 
vided to  form  two  daughter  cells,  these  two  cells  did  not 
move  apart,  but  remained 
side  by  side  and  each  di- 
vided to  form  two  more, 
and  each  of  these  divided 
to  form  two  more,  and 
these  eight  divided  each 
into  two,  each  cell  com- 
plete and  independent  but 
all  remaining  together 
in  a  group  —  if  this  pro- 
cess should  take  place  we 
should  have  produced  a 
group  or  colony  of  sixteen 
cells,  each  cell  a  complete 
animal  capable  of  living 
independently  like  the 
other  simplest  animals, 


but  all  holding  together 


B 


to  form  a  tiny,  flat,  plate-  FIG.  12.— Gonium  pectorale  (after  STEIN).  A, 
like  Colony.  NOW,  this  is  colony  seen  from  above;  B,  colony  seen 

•  7  from  the  side. 

precisely  what  takes  place 

in  the  case  of  those  colonial  Protozoa  belonging  to  the  genus 
Gonium  (Fig.  12).  When  the  mother  cell  of  Gonium  di- 
vides, the  daughter  cells  do  not  swim  apart,  but  remain 
side  by  side,  and  by  repeated  fission,  until  there  are  sixteen 
cells  side  by  side,  the  colony*is  formed.  Each  cell  of  the 


26 


ANIMAL  LIFE 


colony  eats  and  breathes  and  feels  for  itself  ;  each  can  and 
does  perform  all  the  processes  necessary  to  keep  it  alive. 
When  ready  to  multiply,  the  sixteen  cells  of  the  Gonium 
colony  separate,  and  each  cell  becomes  the  ancestor  of  a 
new  colony. 

15.  Pandorina. — Another  colony  usually  composed  of  six- 
teen cells  is  Pandorina,  but  the  cells  are  arranged  to  form 
a  spherical  instead  of  a  plate-like  colony  (Fig.  13).  In  Pan- 
dorina morum  the  colony  consists  of  sixteen  ovoid  cells  in 
a  spherical  jelly-like  mass.  Each  cell  has  two  flagella,  and 
by  the  lashing  of  all  the  flagella  the  whole  colony  moves 
through  the  water.  Food  is  taken  by  any  of  the  cells,  is 
assimilated,  and  the  cells  increase  in  size.  When  Pan- 
dorina is  ready  to  multiply,  each  cell  divides  repeatedly 
until  it  has  formed  sixteen  daughter  cells.  The  inclosing 
gelatinous  mass  which  holds  the  colony  together  dissolves, 

and  the  daughter  colonies  be- 
come free  and  swim  apart. 
Each  colony  soon  grows  to  the 
size  of  the  original  colony. 
This  kind  of  multiplication  or 
reproduction  may  be  continued 
for  several  generations.  But 
it  does  not  go  on  indefinitely. 
After  a  number  of  these  gener- 
ations has  been  produced  by 
simple  division,  the  cells  of  a 
colony  divide  each  into  eight 
instead  of  sixteen  daughter 
cells.  The  daughter  cells  are 
not  all  of  the  same  size,  but 
the  difference  is  hardly  notice- 

The  eight  cells  resulting  from  the  repeated  division 
of  one  of  the  original  cells  separate  and  swim  about  inde- 
pendently by  means  of  their  flagella.  If  one  of  these  cells 
comes  near  a  similar  free-swimming  cell  from  another 


FIG.  13.—  Pandorina  sp.  (from  Na- 
ture). The  cells  composing  the 
colony  are  beginning  to  divide  to 
form  daughter  colonies. 


able. 


THE  LIFE  OF  THE  SLIGHTLY  COMPLEX  ANIMALS       27 


colony,  the  two  cells  conjugate  (Fig.  14) — that  is,  fuse  to 
form  a  single  cell.  This  new  cell  formed  by  the  fusion  of 
two,  develops  a  tough  enveloping  membrane  of  cellulose 
and  passes  into  what  is  called 
the  "  resting  stage."  That  is, 
the  cell  remains  dormant  for  a 
shorter  or  longer  time.  It  may 
thus  tide  over  a  drought  or  a 
winter.  It  may  become  dry  or 
be  frozen,  yet  when  suitable 
conditions  of  moisture  or  tem- 
perature are  again  present  the 
outer  wall  breaks  and  the  pro- 
toplasm issues  as  a  large  free- 
swimming  cell,  which  soon  di- 
vides into  sixteen  daughter 
cells  which  constitute  a  new 
colony. 

16.  Eudorina.— Another  colo- 
nial protozoan  which  much  re- 
sembles Pandorina,  but  differs 
from  it  in  one  interesting  and 
suggestive  thing,  is  Eudorina. 
In  Eudorina  elegans  (Fig.  15) 
the  colony  is  spherical  and  is 
composed  of  sixteen  or  thirty- 
two  cells.  Each  of  these  cells 
can  become  the  parent  of  a  new 
colony  by  simple  repeated  divi- 
sion. But  this  simple  mode  of 
reproduction,  just  as  with  Pan- 
dorina, can  not  persist  indefi- 
nitely. There  must  be  conjuga- 
tion. But  the  process  of  mul- 
tiplication, which  includes  conjugation,  is  different  from 
that  process  in  Pandorina,  in  that  in  Eudorina  the  conju- 


B 


FIG.  14.  —  Pandorina  morum  (after 
GOEBEL).  Three  stages  in  the 
conjugation  and  formation  of  the 
resting  spore.  A,  two  cells  just 
fused;  B,  the  two  cells  completely 
fused,  but  with  flagella  still  per- 
sisting ;  C,  the  resting  spore. 


28 


ANIMAL   LIFE 


gating  cells  are  of  two  distinctly  different  kinds.  When 
this  kind  of  multiplication  is  to  take  place  in  the  case  of 
Eudorina  elegans,  to  choose  a  common  species,  some  of 
the  cells  of  a  colony  divide  into  sixteen  or  thirty -two 

minute  elongated  cells,  each 
provided  with  two  flagella. 
These  small  cells  escape 


FIG.  15.—  Eudorina  elegant.    A,  a  mature  colony  (from  Nature);   B,  formation  of 
the  two  kinds  of  reproductive  cells. 

from  the  envelope  of  the  parent  cell,  remaining  for  some 
time  united  in  small  bundles.  Other  cells  of  the  colony 
do  not  divide,  but  increase  slightly  in  size  and  become 
spherical  in  shape.  When  a  bundle  of  the  small  cells 
comes  into  contact  with  some  of  these  large  spherical 
cells  the  bundle  breaks  up,  and  conjugation  takes  place 
between  the  small  flagellated  free-swimming  cells  and  the 
large  non-flagellate  spherical  cells.  Each  new  cell  formed 
by  the  fusion  of  one  of  the  small  and  one  of  the  large  cells 
develops  a  cellulose  wall  and  assumes  a  resting  stage. 
After  a  time  from  each  of  these  resting  spores  a  new  colony 
of  sixteen  or  thirty-two  cells  is  formed  by  direct,  repeated 
division. 

17.  Volvox. — Another  interesting  colonial  protozoan  is 
Volvox.     The  large  spherical  colonies  of    Volvox  globator 


THE  LIFE  OF  THE  SLIGHTLY  COMPLEX  ANIMALS       29 


(Fig.  16)  are  composed  of  several  thousand  cells,  arranged 

in   a   single  peripheral  layer  about  the  hollow  center  of 

the  ball.     The  cells  are  ovoid,  and  each  is  provided  with 

two  long  flagella  which  pro- 

ject out  into  the  water.  The 

lashing  of  the  thousands  of 

the   flagella  give   the  ball- 

like  colony  a  rotary  motion. 

The  cells  are  held  together 

by  a  jelly-like  intercellular 

substance  and  are  connect- 

ed with  each  other  by  fine 

protoplasmic  threads  which 

extend  from  the  body  pro- 

toplasm of  one  cell  to  the 

cells  surrounding  it.    When 

the  colony  is  full  grown  and 

ready   to    reproduce    itself 

certain  cells  of  the  colony 

undergo      great      changes. 

Some  of  them   increase  in 

size  enormously,  having  re- 

serve food  material   stored 

in  them,  and  they  may  be 

called  the  egg  cells  of  the 

colony.    Eeproduction  may 

now  occur  by  simple  divi- 

sion of  one  of  these  great 

egg  cells  into  many  small 

cells,  all  held  together  in  a 

Common    envelope.        These    Fl«-  16.—  A,  Volvox  minor,   entire   colony 

form    a    daughter    colony 

which    escapes    from    the 

mother  colony  and  by  growth  and  further  division  comes  to 

be  a  new  full-sized  colony.     Or  reproduction  may  occur  in 

another,  more  complex,  way.     Certain  cells  of  the  colony 


B 


£ST  or  "%L 


30  ANIMAL  LIFE 

divide  into  bundles  of  very  small,  slender  cells,  each  of 
which  is  provided  with  flagella.  The  remaining  cells  of 
the  colony  (that  is,  those  which  have  not  swollen  into  egg 
cells  or  divided  into  many — sixty-four  to  one  hundred  and 
twenty-eight — minute,  flagellate  cells)  remain  unchanged  for 
a  while  and  finally  die.  They  take  absolutely  no  part  in 
reproducing  the  colony.  One  of  the  minute  free-swim- 
ming cells  fuses  with  one  of  the  enormous  egg  cells,  the 
new  cell  thus  formed  being  a  resting  spore.  From  this 
resting  spore  a  new  colony  develops  by  repeated  division. 

18.  Steps  toward  complexity.— Within  the  group  of  Vol- 
vocince  there  are  plainly  several  steps  on  the  way  from 
simplicity  of  structure  to  complexity  of  structure.  Gonium, 
Pandorina,  Eudorina^  and  Volvox  form  a  series  proceeding 
from  the  simplest  animals  toward  the  complex  animals. 
In  Gonium  the  cells  composing  the  colony  are  all  alike  in 
structure,  and  each  one  is  capable  of  performing  all  the 
processes  or  functions  of  life.  In  Pandorina  and  Eudorina 
the  cells  are  at  first  alike,  but  there  is,  as  the  time  for 
reproduction  approaches,  a  differentiation  of  structure ; 
the  cells  of  the  colony,  all  of  which  take  part  in  the  process 
of  reproduction,  come  to  be  in  certain  generations  of  two 
kinds — an  inactive  large  kind  which  may  be  called  the  egg 
cells,  and  a  small,  active,  free-swimming  kind  which  seeks 
out  and  conjugates  with,  or,  we  may  say,  fertilizes  the  egg 
cells.  In  Volvox  there  is  a  new  differentiation.  Only  cer- 
tain particular  and  relatively  few  cells  take  part  in  repro- 
ducing the  colony;  most  of  the  cells  have  given  up  the 
power  or  function  of  reproduction.  These  cells,  when  the 
time  of  multiplication  comes,  simply  support  the  special 
reproductive  cells.  They  continue  to  waft  the  great  colony 
through  the  water  by  lashing  their  flagella ;  they  continue 
to  take  in  food  from  the  outside.  The  reproductive  cells 
devote  themselves  wholly  to  the  business  of  producing  new 
colonies,  of  perpetuating  the  species.  And  this  matter  of 
reproduction  is  less  simple  than  in  the  other  Volvocince. 


THE  LIFE  OP  THE  SLIGHTLY  COMPLEX  ANIMALS       31 

At  least  there  is  much  more  difference  between  the  two 
kinds  of  reproductive  cells.  The  egg  cells  are  compara- 
tively enormous,  and  they  are  stored  with  a  mass  of  food 
material.  The  fertilizing  cells  are  very  small,  but  very 
active  and  very  different  from  the  egg  cells.  We  have  in 
Volvox  the  beginnings  of  a  distinct  division  of  labor  and 
an  accompanying  differentiation  of  structure.  Certain 
cells  of  the  colony  do  certain  things,  and  are  modified  in 
structure  to  fit  them  specially  for  their  particular  duties. 
The  steps  from  the  simplest  structure  toward  a  complex 
structure  are  plainly  visible. 

'lO^Individual  or  colony. — Is  the  Gonium  colony,  the 
Pandorina  colony,  or  the  Volvox  colony  a  group  of  several  or 
many  distinct  organisms,  or  is  it  to  be  considered  as  a  sin- 
gle organism  ?  With  Gonium,  which  we  may  call  the  sim- 
plest of  these  colonial  organisms,  the  colony  is  composed 
of  a  few  wholly  similar  cells  or  one-celled  animals,  each 
fully  capable  of  performing  all  the  life  processes,  each 
wholly  competent  to  lead  an  independent  life.  In  fact, 
each  does,  for  part  of  its  life,  live  independently,  as  we 
have  already  described.  In  the  case  of  Pandorina  and  Eu- 
dorina,  while  all  the  cells  are  for  most  of  the  lifetime  of  the 
colony  alike  and  each  is  capable  of  living  independently, 
at  the  time  of  reproduction  the  cells  become  of  two  kinds. 
A  difference  of  structure  is  apparent,  and  for  the  perpetua- 
tion of  the  species  the  co-operation  of  these  different  kinds 
of  cells  is  necessary.  That  is,  it  is  impossible  for  a  single 
one  of  the  members  of  the  colony  to  reproduce  the  colony, 
except  for  a  limited  number  of  generations.  With  Volvox 
this  giving  up  of  independence  on  the  part  of  the  individual 
members  of  the  colony  is  more  marked.  There  is  a  real  in- 
terdependence among  the  thousands  of  cells  of  the  colony. 
The  function  of  reproduction  rests  with  a  few  particular 
cells,  and  for  the  perpetuation  of  the  species  there  is  demand- 
ed a  co-operation  of  two  distinct  kinds  of  reproductive  cells. 
The  great  majority  of  the  cells  take  no  part  in  reproduo 


32  ANIMAL  LIFE 

tion.  They  can  perform  all  the  other  life  processes  ;  they 
move  the  colony  by  lashing  the  water  with  their  flagella ; 
they  take  in  food  and  assimilate  it ;  they  can  feel.  All  the 
cells  of  the  great  colony,  too,  are  intimately  connected  by 
means  of  protoplasmic  threads.  The  protoplasm  of  one 
cell  can  mingle  with  that  of  another  cell;  food  can  go 
from  cell  to  cell.  The  question  whether  the  Volvox  colony 
is  a  group  of  distinct  organisms  or  is  a  single  organism 
made  up  of  cells  among  which  there  is  a  simple  but  obvi- 
ous difference  in  structure  and  function  ;  in  other  words, 
whether  Volvox  is  a  colony  of  one-celled  animals,  of  Pro- 
tozoa, or  is  a  multicellular  animal,  one  of  the  Metazoa  (for 
so  all  the  many-celled  animals  are  called),  is  a  difficult  one 
to  decide.  Most  zoologists  class  the  Volvocinae  with  the 
Protozoa — that  is,  they  incline  to  consider  Gonium,  Pan- 
dorina,  Volvox,  and  the  other  Volvocinae  as  groups  or  col- 
onies of  one-celled  animals. 

20.  Sponges. — If  the  VolvocincB  be  considered  to  belong 
to  the  Protozoa,  the  sponges  are  the  simplest  of  all  the 
many-celled  animals.  Sponges  are  not  free-swimming  ani- 
mals, except  for  a  short  time  in  their  young  stage,  but  are 
fixed,  like  plants.  They  live  attached  to  some  solid  sub- 
stance on  the  sea  bottom.  They  resemble  plants,  too,  in 
the  way  in  which  the  body  is  modified  during  growth  by 
the  environment.  If  the  rock  to  which  the  young  sponge 
is  attached  is  rough  and  uneven,  the  body  of  the  sponge 
will  grow  so  as  to  fit  the  unevenness  ;  if  the  rock  surface  is 
smooth,  the  body  of  the  sponge  will  be  more  regular.  Thus 
a  sponge  may  be  said  to  have  no  fixed  shape  of  body  ;  indi- 
viduals of  the  same  species  of  sponge  differ  much  in  form. 
The  typical  form  of  the  sponges  is  that  of  a  short  cylinder 
or  vase  attached  by  one  end  and  with  the  upper  free  end 
open  (Fig.  17).  Many  individuals  of  one  kind  usually  live 
together  in  a  close  group  or  colony,  and  they  may  be  so 
attached  to  each  other  as  to  appear  like  a  branching  plant. 
This  branching  may  be  very  diffuse,  and  the  branches 


THE  LIFE  OF  THE  SLIGHTLY  COMPLEX  ANIMALS       33 


may  become  so  interwoven  with  each  other  as  to  form  a 
very  complex  group.  A  sponge  is  composed  of  many  cells 
arranged  in  three  layers — that  is,  the  body  of  a  sponge  is  a 
cylinder  closed  at  one  end  whose  wall  is  composed  of  three 
layers  of  cells.  The  outer  layer  of 
cells  is  called  the  ectoderm,  and  the 
cells  composing  it  are  flat  and  are 
all  closely  attached  to  each  other. 
The  inner  layer  is  called  the  endo- 
derm,  and  its  cells  are  thicker  than 
those  of  the  ectoderm  ;  they  are 
also  closely  attached  to  each  other. 
Sometimes  they  are  provided  with 
flagella  like  the  flagellate  Protozoa. 
The  flagella  are,  however,  not  for  the 
purpose  of  locomotion,  but  for  creat- 
ing currents  in  the  water,  which 
bathes  the  interior  of  the  open  cylin- 
drical body.  The  middle  layer, 
called  the  mesoderm,  is  composed  of 
numerous  separate  cells  lying  in  a 
jelly-like  matrix.  From  these  meso- 
derm cells  fine  needles  or  spicules 
of  lime  or  silica  often  project  out 
through  the  ectoderm.  These  mi- 
nute sponge  spicules  are  of  a  great 
variety  of  shapes,  and  they  form  a 
sort  of  skeleton  for  the  support  of  F™- 
the  soft  body  mass.  All  over  the 
outer  surface  of  the  body  are  scat- 
tered fine  openings  or  pores,  which 
lead  through  the  walls  of  the  body 

into  the  inner  cavity.  This  cavity  is  of  course  also  con- 
nected with  the  outside  by  the  large  opening  at  the  free  or 
apical  end  of  the  body. 

There  is  hardly  any  differentiation  of  parts  among  the 
4 


of  the  simplest 
sponges,  Calcolynthus  pri- 
migenlus  (after  HAECKEL). 
A  part  of  the  outer  wall  is 
cut  away  to  show  the  in- 
side. 


ANIMAL  LIFE 


sponges.  As  in  the  Protozoa,  there  are  no  special  organs 
for  the  performance  of  special  functions.  The  sponge 
feeds  by  creating,  with  its  flagella,  water  currents  which 

flow  in  through  the  many  fine 
pores  of  the  body  and  out  from 
the  inner  body  cavity  through 
the  large  opening  at  the  free 
end  of  the  body.  These  cur- 
rents of  water  bear  fine  parti- 
cles of  organic  matter  which 
are  taken  up  by  the  cells  lining 
the  pores  and  body  cavity,  and 
assimilated.  There  are  no 
special  organs  of  digestion. 
Each  cell  takes  up  food  and 
digests  it.  The  water  cur- 
rents also  bring  air  to  these 
same  cells,  and  thus  the  sponge 
breathes.  Although  the 
sponge  as  a  whole  can  not 
move,  does  not  possess  the 
power  of  locomotion,  yet  the 
protoplasm  of  the  cells  has 
the  power  of  contracting,  just 
as  with  the  Protozoa,  and  the 
pores  can  be  opened  or  closed 
by  this  cellular  movement. 
Practically,  thus,  the  only 
movements  the  sponge  can 
.  Thebody  is  represented  make  are  the  movements  made 
as  cut  in  two  longitudinally.  The  by  the  individual  cells. 

large  cells  of  the  inner  layer  are  the 

egg  cells.  Eeproduction     is     accom- 

plished by  a  process  of  divi- 
sion, or  by  a  process  of  conjugation  and  subsequent  division. 
In  its  simplest  way  multiplication  takes  place  by  a  group 
of  cells  separating  from  the  body  of  the  parent  sponge, 


PIG.  18. — One  of   the  simple   sponges, 
Prophysema      primordiale     (after 


THE  LIFE  OF  THE  SLIGHTLY  COMPLEX  ANIMALS       35 

becoming  inclosed  in  a  common  capsular  envelope,  and  by 
repeated  division  and  consequent  increase  in  number  of 
cells  becoming  a  new  sponge.  This  is  reproduction  by 
"  budding."  The  "  buds,"  or  small  groups  of  cells  which 
separate  from  the  parent  sponge,  are  called  gemmules. 
Reproduction  in  the  more  complex  way  occurs  as  follows  : 
Some  of  the  free  amoeboid  cells  of  the  mesoderm  (the  mid- 
dle one  of  the  three  layers  of  the  body  wall)  become  en- 
larged and  spherical  in  form.  These  are  the  egg  cells. 
Other  mesodermic  cells  divide  into  many  small  cells,  which 
are  oval  with  a  long,  tapering,  tail-like  projection.  These 
cells  are  active,  being  able  to  swim  by  the  lashing  of  the 
tapering  tail.  These  are  the  fertilizing  cells.  The  two 
kinds  of  reproductive  cells  may  be  formed  in  one  sponge ; 
if  so,  they  are  formed  at  different  times.  Or  one  sponge 
may  produce  only  egg  cells,  another  only  fertilizing  or, 
as  they  are  called,  sperm  cells.  Conjugation  takes  place 
between  a  sperm  cell  and  an  egg  cell.  That  is,  one  of  the 
small  active  sperm  cells  finds  one  of  the  large,  spherical, 
inactive  cells  and  penetrates  into  the  protoplasm  of  its 
body.  The  two  cells  fuse  and  form  a  single  cell,  which 
may  be  called  the  fertilized  or  impregnated  egg.  This  fer- 
tilized egg,  remaining  in  the  body  mass  of  the  parent 
sponge,  divides  repeatedly,  the  new  cells  formed  by  this 
division  remaining  together.  The  young  or  embryo  sponge 
finally  escapes  from  the  body  of  the  parent  sponge,  and 
lives  for  a  short  time  as  an  active  free-swimming  animal. 
Its  body  consists  of  an  oval  mass  of  cells,  of  which  those  on 
one  side  are  provided  with  cilia  or  swimming  hairs.  The 
cells  of  the  body  continue  to  divide  and  to  grow,  and  the 
body  shape  gradually  changes.  The  young  sponge  finally 
becomes  attached  to  some  rock,  the  body  assumes  the  typi- 
cal cylindrical  shape,  an  aperture  appears  at  the  free  end, 
and  small  perforations  appear  on  the  surface.  The  sponge 
becomes  full  grown. 

Those  of  us  who  do  not  live  in  the  vicinity  of  the  sea- 


36  ANIMAL  LIFE 

shore  where  sponges  are  found  can  not  observe  the  struc- 
ture and  life  history  of  living  specimens.  There  are,  how- 
ever, among  the  thousand  and  more  kinds  of  sponges  a  few 
kinds  that  live  in  fresh  water,  and  these  are  so  widely 
spread  over  the  earth  that  examples  of  them  can  be  found 
in  almost  any  region.  They  belong  to  the  genus  Spongilla, 
and  thirty  or  more  species  or  kinds  of  Sponyilla  are  known. 
In  standing  or  slowly  flowing  water,  Spongilla  grows  erect 
and  branching,  like  a  shrub  or  miniature  tree ;  in  swift 
water  it  grows  low  and  spreading,  forming  a  sort  of  mat 
over  the  surface  to  which  it  is  attached.  Eeproduction 
takes  place  very  actively  by  the  process  of  budding.  The 
budded-off  gemmules  are  spherical  in  shape,  and  the  cells 
of  each  gemmule  are  inclosed  in  an  envelope  composed  of 
siliceous  spicules  of  peculiar  shape.  These  gemmules  are 
formed  in  the  body  substance  of  the  parent  sponge  toward 
the  end  of  the  year,  and  are  set  free  by  the  decaying  of 
that  part  of  the  body  of  the  parent  sponge  in  which  they 
lie.  They  sink  to  the  bottom  of  the  pond  or  brook,  and 
lie  there  dormant  until  the  following  spring.  Then  they 
develop  rapidly  by  repeated  division  of  the  cells  and 
growth. 

It  is  not  the  purpose  here  to  describe  the  many  and 
interesting  kinds  of  sponges  which  inhabit  the  ocean.  The 
sponge  of  the  bathroom  is  simply  the  skeleton  of  a  large 
sponge  or  group  of  sponges.  The  skeleton  here  is  not 
composed  of  lime  or  silica,  but  of  a  tough,  horny  substance, 
which  is  secreted  by  cells  of  the  mesodermal  layer  of  the 
body  wall  of  the  sponge.  This  substance  is  called  spongin, 
and  is  a  substance  allied  to  silk  in  its  chemical  composi- 
tion. All  the  commercial  sponges,  the  spongin  skeletons, 
belong  to  one  genus — Spongia.  These  sponges  grow  espe- 
cially abundantly  in  the  Mediterranean  and  Eed  Seas,  and 
in  the  Atlantic  Ocean  off  the  Florida  reefs,  and  on  the 
shores  of  the  Bahama  Islands.  The  sponges  are  pulled 
up  by  divers,  or  by  means  of  hooks  or  dredges.  The 


THE  LIFE  OF  THE  SLIGHTLY  COMPLEX  ANIMALS      37 

living  matter  soon  dies  and  decays,  leaving  the  horny 
skeleton,  which  when  cleaned  and  trimmed  is  ready  for 
use. 

The  most  beautiful  sponges  are  those  with  siliceous 
skeletons.  The  fine  needles  or  threads  of  glass,  arranged 
often  in  delicate  and  intricate  pattern,  make  these  sponges 
objects  of  real  beauty. 

21.  Polyps,  corals,  and  jelly-fishes. — The  general  or  typ- 
ical plan  of  body  structure  of  those  animals  which  come 
next  in  degree  of  complexity  to  the  sponges  can  be  best 
understood  by  imagining  the  typical  cylindrical  body  of  a 
sponge  modified  in  the  following  way:   The    middle  one 
of  the  three  layers  of  the  body  wall  not  to  be  composed 
of  cells  in  a  gelatinous  mass,  but  to  be  simply  a  thin  non- 
cellular    membrane;   the  body  wall  to  be  pierced  by  no 
fine  openings  or  pores,  so  that  the  interior  cavity  of  the 
body  is   connected   with  the  outside  only  by  the   single 
large  opening  at  the  free  end,  and  this  opening  to  be  sur- 
rounded by  a  circlet  of  arm-like  processes  or   tentacles, 
continuations   of  the  body  wall  and  similarly  composed. 
Such  a  body  structure  is  the  general  or  fundamental  one 
for  all  polyps,  corals,  sea-anemones,  and  jelly-fishes.     The 
variety  in  shape  and  the  superficial  modifications  of  this 
type-plan  are  many  and  striking ;  but,  after  all,  the  type- 
plan  is  recognizable  throughout  the  whole  of  this  great 
group  of  animals.     Perhaps  the  simplest  representative  of 
the  group  is  a  tiny  polyp  which  grows  abundantly  in  the 
fresh-water  streams  and  pools,  and  can  be  readily  obtained 
for  observation.     It  is  called  Hydra. 

22.  Hydra,— The  body  of   Hydra  (Fig.   19),  which  is 
very  small  and  appears  to  the  unaided  eye  as  a  tiny  white 
or  greenish  gelatinous  particle  attached  to  some  submerged 
stone,  bit  of  wood,  or  aquatic  plant,  is  a  simple  cylinder 
attached  by  one  end  to  the  stone  or  weed.     The  other  free 
end  is  contracted  so  as  to  be  conical,  and  it  is  narrowly 
open.     Around  the  opening  are  six  or  eight  small  waving 


38  ANIMAL  LIFE 

tentacles.  The  wall  of  the  cylinder  is  composed  of  an 
outer  and  an  inner  layer  of  cells  and  a  thin  non-cellular 
membranous  layer  between  them.  The  tentacles  are  hol- 
low and  are  simple  extensions  of  the  body  wall.  The  cells 
of  the  outer  layer,  or  ectoderm,  are  not  all  alike.  Some 
are  smaller  than  the  others  and  appear  to  be  crowded  in 


FIG.  19.— The  fresh- water  polyp,  Hydra  vulgaris.  A,  in  expanded  condition,  and 
in  contracted  condition;  B,  cross  section  of  body,  showing  the  two  layers  of 
cells  which  make  up  the  body  wall. 

between  the  bases  or  inner  ends  of  the  larger  ones.  The 
inner  ends  of  the  large  cells  are  extended  as  narrow-pointed 
prolongations  directed  at  right  angles  with  the  rest  of  the 
cell.  These  processes  are  very  contractile  and  are  called 
muscle  processes.  Each  one  is  simply  a  continuation  of 
the  protoplasm  of  the  cell  body,  which  is  especially  con- 
tractile. Some  of  the  smaller  ectoderm  cells  are  very 


THE  LIFE  OF  THE  SLIGHTLY  COMPLEX  ANIMALS       39 

irregular  in  shape  and  possess  specially  large  nuclei.  These 
cells  are  more  irritable  or  sensitive  than  the  others  and 
are  called  nerve  cells.  The  ectoderm  cells  of  the  base  or 
foot  of  the  Hydra  are  peculiarly  granular,  and  secrete  a 
sticky  substance  by  which  the  Hydra  holds  fast  to  the 
stone  or  weed  on  which  it  is  found.  These  cells  are  called 
gland  cells.  Imbedded  in  many  of  the  larger  ectoderm 
cells,  especially  those  of  the  tentacles,  are  small  oval  sacs, 
in  each  of  which  lies  folded  or  coiled  a  fine  long  thread. 
When  the  tentacles  touch  one  of  the  small  animals  which 
serve  Hydra  as  food,  these  fine  threads  shoot  out  from 
their  sacs  and  so  poison  or  sting  the  prey  that  it  is 
paralyzed.  The  tentacles  then  contract  and  bend  inward, 
forcing  the  captured  animal  into  the  mouth  opening 
in  the  center  of  the  circle  of  tentacles.  Through  the 
mouth  opening  the  prey  enters  the  body  cavity  of  Hydra 
and  is  digested  by  the  cells  lining  this  cavity.  These  cells 
belonging  to  the  inner  layer  of  the  body  wall  or  endoderm 
are  mostly  large,  and  each  contains  one  or  more  contractile 
vacuoles.  From  the  free  ends — the  ends  which  are  next  to 
the  body  cavity — of  these  cells  project  pseudopods  or  fine 
flagella.  These  projections  are  constantly  changing :  now 
two  or  three  short,  blunt  pseudopods  are  projecting  into 
the  body  cavity ;  now  they  are  withdrawn,  and  a  few  fine, 
long  flagella  are  projected.  In  addition  to  these  cells  there 
are  in  the  endoderm,  especially  abundant  near  the  mouth 
opening  and  wholly  lacking  in  the  tentacles  and  at  the 
base  of  the  body,  many  long,  narrow,  granular  cells.  They 
are  gland  cells  which  secrete  a  digestive  fluid.  The  food 
captured  by  the  tentacles  and  taken  in  through  the 
mouth  opening  disintegrates  in  the  body  cavity,  or  diges- 
tive cavity,  as  it  may  be  called.  The  digestive  fluid  se- 
creted by  the  gland  cells  of  the  endoderm  acts  upon  it, 
so  that  it  becomes  broken  into  small  parts.  These  par- 
ticles are  probably  seized  by  the  pseudopods  of  the  other 
endoderm  cells  and  are  taken  into  the  body  protoplasm 


40  ANIMAL  LIFE 

of  these  cells.  The  ectoderm  cells  do  not  take  food 
directly,  but  receive  nourishment  only  through  the  endo- 
derm  cells. 

Hydra  is  not  permanently  attached.  It  holds  firmly 
to  the  submerged  stone  or  weed  by  means  of  the  sticky 
secretion  from  the  ectodermal  gland  cells  of  its  base,  but  it 
can  loosen  itself,  and  by  a  slow  creeping  or  gliding  move 
along  the  surface  of  the  stone  to  another  spot.  Even  when 
attached,  the  form  of  the  body  changes ;  it  extends  itself 
longitudinally,  or  it  contracts  into  a  compact  globular  mass. 
The  tentacles  move  about  in  the  water,  and  are  continually 
contracting  or  extending. 

Like  Volvox  and  the  sponges,  those  other  slightly  com- 
plex animals  we  have  already  considered,  Hydra  has  two 
methods  of  multiplication.  In  the  simpler  way,  there 
appears  on  the  outer  surface  of  the  body  a  little  bud  which 
is  composed,  at  first,  of  ectoderm  cells  alone ;  but  soon  it  is 
evident  that  it  is  a  budding,  or  outpushing,  of  the  whole 
body  wall,  ectoderm,  endoderm,  and  middle  membrane.  In 
a  few  hours  the  bud  has  six  or  eight  tiny,  blunt  tentacles, 
a  mouth  opening  appears  at  the  free  end,  and  the  little 
Hydra  breaks  off  from  the  parent  body  and  leads  an  inde- 
pendent existence.  In  the  more  complex  way,  two  kinds  of 
special  reproductive  cells  are  produced  by  each  individual, 
viz.,  large,  inactive,  spherical  egg  cells,  and  small,  active 
sperm  cells,  each  with  an  oval  part  or  head  (consisting  of 
the  nucleus)  and  a  slender,  tapering  tail-like  part  (consist- 
ing of  the  cytoplasm).  The  egg  cell  lies  inclosed  in  a  layer 
of  thin,  surrounding  cells,  which  compose  a  capsule  for  it. 
When  the  egg  cell  is  ready  for  fertilization  this  capsule 
breaks,  and  one  of  the  active  sperm  cells  finds  its  way  to 
and  fuses  with  the  egg  cell.  The  fertilized  egg  cell  now 
divides  into  several  cells,  which  remain  together.  The 
outer  ones  form  a  hard  capsule,  and  thus  protected  the 
embryo  falls  to  the  bottom,  and  after  lying  dormant  for 
awhile  develops  into  a  Hydra. 


THE  LIFE  OF  THE  SLIGHTLY  COMPLEX  ANIMALS      41 

23.  Differentiation  of  the  body  cells. — In  Hydra  we  have 
the  beginnings  of  complexity  of  structure  carried  a  step 
further  than  in  the  sponges.     The  division  of  labor  among 
the  cells  composing  the  body  is  more  pronounced,  and  the 
structural  modification  of  the  different  cells  to  enable  them 
better  to  perform  their  special  duties  is  obvious.     Some  of 
the  cells  of  the  body  specially  devote  themselves  to  food- 
taking  ;  some  specially  to  the  digestion  of  the  food ;  some 
are  specially  contractile,  and  on  them  the  movements  of 
the  body  depend,  while   others  are   specially  irritable  or 
sensitive,  and  on  them  the  body  depends  for  knowledge  of 
the  contact  of  prey  or  enemies.     In  the  lasso  cells — those 
with  the  stinging  threads — there  is  a  very  wide  departure 
from  the  simple  primitive  type  of  cells.    There  is  in  Hydra 
a  manifest  differentiation  of  the  cells  into  various  kinds  of 
cells.     The  beginnings  of  distinct  tissues  and  organs  are 
foreshadowed. 

The  individuals  of  Hydra  live,  usually,  distinct  from 
each  other.  There  is  no  tree-like  colony,  as  with  the  sponges. 
But  most  of  the  other  polyps  do  live  in  this  colonial  manner. 
The  new  polyps  which  develop  as  buds  from  the  body  of 
the  parent  do  not  separate  from  the  parent,  but  remain 
attached  by  their  bases.  They,  in  turn,  produce  new 
polyps  which  remain  attached,  so  that  in  time  a  branching, 
tree-like  colony  is  formed. 

24.  Medusae  or  jelly-fishes. — Most    of  the  other  polyps 
differ  from  Hydra  also  in  producing,  in  addition  to  ordi- 
nary polyp  buds,  buds  which  develop  into  bell-shaped  struc- 
tures called  medusae  (Fig.  20).     These  medusae  consist  of  a 
soft  gelatinous  bell-  or  umbrella-shaped  body,  with  a  short 
clapper   or  stem  which   has  an  opening  at  its  free  end. 
From  the  edge  of  the  bell  or  umbrella  four  pairs  of  tenta- 
cles project.     The  medusae  usually  separate  from  the  parent 
polyp  and  live  an  independent,  free-swimming  life.     These 
are  the  beautiful  animals  commonly  known  as  jelly-fishes. 
The  medusae  or  jelly-fishes  produce  special  reproductive 


ANIMAL  LIFE 


cells,  a  single  medusa  producing  only  one  kind  of  such  cells 
—that  is,  producing  either  egg  cells  alone  or  sperm  cells 
alone.  The  active  sperm  cells  produced  by  one  medusa 
find  their  way  to  an  egg  cell  producing  medusa,  and  fuse 
with  or  fertilize  these  egg  cells.  The 
fertilized  egg  develops  into  a  small, 
oval,  free-swimming  embryo  called  a 
planula,  which  finally  attaches  itself 
to  a  stone  or  bit  of  wood  or  seaweed, 
and  grows  to  be  a  simple  cylindrical 
polyp  attached  at  its  base  and  with 
mouth  and  tentacles  at  its  free  end. 
This  polyp  gives  rise  by  budding  to 
new  polyps,  which  remain  attached 
to  it,  and  gradually  a  new  tree-like 
colony  is  formed.  From  this  polyp 
or  this  colony  new  medusae  bud  off, 
swim  away,  and  finally  produce  new 
polyps.  Thus  there  is  in  the  life  of 
the  polyps  what  is  called  an  alterna- 
There  are  two  kinds  of  individuals 
which  evidently  belong  to  the  same  species  of  animal,  or, 
put  in  another  way,  one  kind  of  animal  has  two  distinct 
forms.  This  appearance  of  one  kind  of  animal  in  two 
forms  is  called  dimorphism.  We  shall  see  later  that  one 
kind  of  animal  may  appear  in  more  than  two  forms  ;  such 
a  condition  is  called  polymorphism.  In  alternation  of  gen- 
erations we  have  the  polyp  animal  appearing  in  one  genera- 
tion as  a  fixed  cylindrical  polyp,  while  in  the  next  generation 
it  is  a  free-swimming,  umbrella-shaped  medusa  or  jelly-fish. 
The  polyps  which  are  dimorphic — that  is,  have  a  polyp 
form  of  individual  and  a  medusa  form  of  individual — show 
more  differentiation  in  structure  than  the  simple  Hydra. 
This  further  differentiation  is  especially  apparent  in  the 
medusae  or  jelly-fishes.  Here  the  nerve  cells  are  aggregated 
in  little  groups  arranged  along  the  edge  of  the  umbrella 


FIG.  20.— A  medusa,  Eucope. 
—After  HAKCKEL. 

tion  of  generations. 


THE  LIFE  OF  THE  SLIGHTLY  COMPLEX  ANIMALS      43 

to  form  distinct  sense  organs.  The  muscle  processes  are 
better  developed,  and  the  digestive  cavity  is  differentiated 
into  central  and  peripheral  portions.  In  these  dimorphic 
polyps  the  fixed  polyp  individuals  reproduce  by  the  simple 
way  of  budding,  while  the  medusa  individuals  reproduce 
by  producing  special  reproductive  cells  of  two  kinds,  which 
must  fuse  to  form  a  cell  capable  of  developing  into  a  new 
polyp. 

25.  Corals. — There  are  many  kinds  of  polyps  and  jelly- 
fishes,  and  they  present  a  great  variety  of  shape  and  size 
and  general  appearance.  Many  polyps  exist  only  in  the 
true  polyp  form,  never  producing  medusae.  Others  have 


PIG.  21. — A  polyp,  or  sea-anemone  (Metridium  dianthus). 

only  the  medusa  form.  Some  live  in  colonies,  and  others 
are  always  solitary.  The  animals  we  know  as  corals  are 
polyps  which  live  in  enormous  colonies,  and  which  exist 
only  in  the  true  polyp  form,  not  producing  medusas.  They 


PIG.  22.— Coral  island  (Nanuku  Levu,  of  the  Fiji  group).    (After  a  photograph 
by  MAX  AGASSIZ.) 


FIG.  23.— Shore  of  a  coral  island,  with  cocoanut  palms.    (After  a  photograph.) 


THE  LIFE  OF  THE  SLIGHTLY  COMPLEX  ANIMALS      45 

form  a  firm  skeleton  of  lime  (calcium  carbonate),  and  after 
their  death  these  skeletons  persist,  and  because  of  their 
abundance  and  close  massing  form  great  reefs  or  banks  and 
islands.  Coral  islands  occur  only  in  the  warmer  oceans. 
In  the  Atlantic  they  are  found  along  the  coasts  of  southern 
Florida,  Brazil,  and  the  West  Indies ;  in  the  Pacific  and 
Indian  Oceans  there  are  great  coral  reefs  on  the  coast  of 
Australia,  Madagascar,  and  elsewhere,  and  certain  large 


FIG.  24.— Organ-pipe  coral. 

groups  of  inhabited  islands  like  the  Fiji,  Society,  and 
Friendly  Islands  are  composed  exclusively  of  coral  islands. 
More  than  two  thousand  kinds  of  living  corals  are  known, 
and  their  skeletons  offer  much  variety  in  structure  and 
appearance.  Brain  coral,  organ-pipe  coral  (Fig.  24),  the 
well-known  red  coral  from  Italy  and  Sicily,  used  as  jewelry, 
and  the  sea  pens  and  sea  fans  are  among  the  better  known 
and  more  beautiful  kinds  of  coral  skeletons. 

26.  Colonial  jelly-fishes. — While  many  of  the  medusae  or 
jelly-fishes  are  another  form  of  individual  of  a  true  fixed 
polyp,  many  of  the  larger  and  more  beautiful  jelly-fishes  do 
not  exist  in  any  other  form.  Some  of  these  larger  jelly- 
fishes  are  several  feet  in  diameter,  and  when  cast  up  on  the 
beach  form  a  great  shapeless  mass  of  soft,  jelly-like  sub- 


46  ANIMAL  LIFE 

stance.  The  bodies  of  all  jelly-fishes  are  soft  and  gelatinous, 
the  body  substance  containing  hardly  one  per  cent  of  solid 
matter.  It  is  mostly  water.  Many  jelly-fishes  are  beauti- 
fully and  strikingly  colored,  and  as  they  swim  slowly  about 
near  the  surface  of  the  ocean,  lazily  opening  and  shutting 
their  iridescent,  umbrella-like  bodies,  they  are  among  the 
most  beautiful  of  marine  organisms.  When  one  of  the 
jelly-fishes  is  taken  from  the  water,  however,  it  quickly  loses 
its  brilliant  colors,  and  dries  away  to  a  shapeless,  shrivel- 
ing, sticky  mass. 

Some  of  the  most  beautiful  of  the  jelly-fishes  belong 
to  a  group  called  the  Siphonophora.  These  jelly-fishes  are 
elongate  and  tube-like  rather  than  umbrella-  or  bell-shaped, 
and  they  are  polymorphic — that  is,  there  are  several  dif- 
ferent forms  of  individuals  belonging  to  a  single  kind 
or  species.  The  Siphonophora  are  all  free-swimming,  but 
nevertheless  form  small  colonies.  In  the  Mediterranean 
Sea  and  in  other  southern  ocean  waters  the  surface  may  be 
covered  for  great  areas  by  these  brilliantly  colored  jelly-fish 
colonies,  each  of  which  looks,  as  a  celebrated  German  natu- 
ralist has  said,  like  a  swimming  flower  cluster  whose  parts, 
flowers,  stems,  and  leaves  seem  to  be  made  of  transparent 
crystal,  but  which  possess  the  life  and  soul  of  an  animal. 
An  abundant  species  of  these  Siphonophora  (Fig.  25)  is  com- 
posed of  a  slender,  flexible,  floating,  central  stem  several  feet 
long,  to  which  are  attached  thousands  of  medusa  and  polyp 
individuals  representing  several  different  kinds  of  forms, 
each  kind  of  individual  being  specially  modified  or  adapted 
to  perform  some  one  duty.  The  central  stem  is  a  greatly 
elongated  polyp  individual,  whose  upper  end  is  dilated  and 
filled  with  air  to  form  a  float.  This  individual  holds  up 
the  whole  colony.  Grouped  around  this  central  stem  just 
below  the  float  are  many  bell-shaped  bodies  which  alter- 
nately open  and  close,  and  by  thus  drawing  in  and  expelling 
water  from  their  cavities  impel  the  whole  colony  through 
the  water.  These  bell-shaped  structures  are  attached  me- 


THE  LIFE  OF  THE  SLIGHTLY  COMPLEX  ANIMALS      47 


dusa  individuals,  whose 
business  it  is  to  be  the 
locomotive  organs  for  the 
colony.  These  medusae 
are  without  tentacles,  and 
take  no  food  and  produce 
no  young.  They  have 
given  up  the  power  of 
performing  these  other 
life  processes,  and  devote 
themselves  wholly  to  the 
business  of  locomotion. 
From  the  lower  end  of  the 
central  stem  rises  a  host  of 
structures,  among  which 
several  distinct  kinds  are 
readily  perceived.  One 
kind  is  composed  of  a  pear- 
shaped  hollow  body  open 
at  its  free  end,  and  bear- 
ing a  long  tentacle  which 
is  furnished  with  numer- 
ous groups  of  stinging 
cells.  These  are  the  polyp 
individuals  whose  especial 
business  it  is  to  capture 
and  sting  prey  and  to  eat 
it.  These  individuals  are 
the  food -getters  for  the 
colony.  Scattered  among 
these  stinging,  feeding 
polyps,  are  numerous 
smaller  individuals  with 
oval,  closed  body,  each 
bearing  a  long,  slender 
thread.  These  threads 


FIG.  25.— A  colonial  jelly-fish,  Phys&phora 
(after  HAECKEL).  At  the  top  is  the  float 
polyp,  around  its  stem  the  swimming 
medusa1,  and  below  are  the  feeding,  feel- 
ing, protecting,  and  reproducing  polyps 
and  medusifi. 


48  ANIMAL  LIFE 

are  very  sensitive,  and  the  polyps  bearing  them  have  for 
special  function  that  of  feeling  or  being  sensible  of  stimuli 
from  without.  They  are  the  sense  organs  or  sense  indi- 
viduals of  the  colony.  Finally,  there  are  two  other  kinds 
of  structures  or  individuals  which  produce  the  special 
reproductive  cells  for  the  perpetuation  of  the  species. 
These  are  the  modified  medusa  individuals,  and  one  kind, 
larger  than  the  other,  produces  the  active  sperm  cells, 
while  the  other  produces  the  inactive  egg  cells. 

27.  Increase  in  the  degree  of  complexity. — In  the  corals, 
sea-anemones,  and  jelly-fishes  there  is  plainly  much  more 
of  a  division  of  labor  among  the  various  parts  of  an  indi- 
vidual and  much  more  modification  of  these  parts— that  is, 
much  more  structural  complexity  than  among  the  sponges 
and  Hydra.  And  these,  in  their  turn,  are  more  complex  than 
are  the  colonial  Protozoa,  the  Volvocinae.  There  is  a  great 
difference  in  degree  of  complexity  among  the  slightly  com- 
plex animals.  But  the  various  groups  of  these  animals 
which  we  have  studied  can  all  be  arranged  roughly  in  a 
series  beginning  with  the  least  complex  among  them  and 
ascending  to  the  most  complex.  And  in  this  series,  and 
in  the  always  accompanying  division  of  labor  among  the 
different  parts,  the  gradual  increase  in  complexity  is  beau- 
tifully shown. 

From  an  animal  composed  of  many  structurally  simi- 
lar cells,  each  cell  capable  of  performing  all  the  life  pro- 
cesses, we  pass  to  an  animal  composed  of  cells  of  a  few 
different  kinds,  of  slight  structural  diversity.  Each  kind 
of  cell  devotes  itself  especially  to  a  certain  few  life  pro- 
cesses or  functions.  Next  we  find  an  animal  in  which  the 
cells  of  one  kind  are  specially  aggregated  to  form  a  single 
part  of  the  body  which  is  specially  devoted  to  the  perform- 
ance of  a  single  function.  This  diversity  among  the  cells 
increases,  this  aggregation  of  similar  cells  to  form  special 
parts  or  organs  increases,  and  the  division  of  labor  or 
assignment  of  special  functions  to  special  organs  becomes 


THE  LIFE  OF  THE  SLIGHTLY  COMPLEX  ANIMALS      49 

more  and  more  pronounced.  Among  the  more  complex 
polyps  and  jelly-fishes  the  contractile  cells  form  distinct 
muscle  fibers  and  muscles ;  the  sensitive  cells  form  dis- 
tinct nerve  cells  and  nerve  fibers  which  are  arranged  in  a 
primitive  nervous  system ;  the  digestive  cavity  becomes 
complex  and  composed  of  different  portions  ;  the  reproduc- 
tive cells  are  formed  by  special  organs,  and  the  distinction 
between  the  egg  cells  and  the  sperm  cells — that  is,  be- 
tween the  female  reproductive  elements  and  the  male 
reproductive  elements — becomes  more  pronounced. 

We  have  followed  this  increase  or  development  of  struc- 
tural and  physiological  complexity  from  simplest  animals 
to  fairly  complex  ones.  The  principle  of  this  development 
of  complexity  is  evident.  It  will  not  be  profitable  to  at- 
tempt to  follow  in  detail  this  development  among  the 
higher  animals.  The  complex  animals  are  complex  be- 
cause their  life  processes  are  performed  by  special  parts  of 
their  body,  which  parts  are  specially  modified  so  as  to  perf  orrfi 
these  processes  well.  The  animals  which  are  more  complex 
than  those  we  have  studied  differ  from  these  simply  in  the 
degree  of  complexity  attained.  In  order  to  understand 
this  better  we  shall  not  further  consider  special  groups  of 
animals,  but  special  processes  or  functions,  and  attempt  to 
see  how  the  modification  and  increase  in  complexity  of 
structure  goes  hand  in  hand  with  the  increase  of  elaborate- 
ness or  complexity  in  the  performance  of  function. 


CHAPTER  III 

THE  MULTIPLICATION  OF  ANIMALS  AND   SEX 

28.  All  life  from  life. — On  the  performance  of  the  f unc- 
tion of  reproduction  or  multiplication  depends  the  exist- 
ence or  perpetuation  of  the  species.  Although  an  animal 
may  take  food  and  perform  all  the  functions  necessary  to 
its  own  life,  it  does  not  fulfill  the  demands  of  successful 
existence  unless  it  reproduces  itself.  Some  individuals  of 
every  species  must  produce  offspring  or  the  species  becomes 
extinct.  We  have  seen  in  our  study  of  the  simple  animals 
that  the  function  of  reproduction  is  the  first  function  to 
become  differentiated  in  the  ascent  from  simplest  animals 
to  complex  animals.  The  first  division  of  labor  among  the 
cells  composing  the  bodies  of  the  slightly  complex  animals 
and  the  first  structural  differences  among  the  cells  are 
connected  with  the  performance  of  the  function  of  repro- 
duction or  multiplication. 

We  are  all  so  familiar  with  the  fact  that  a  kitten 
comes  into  the  world  only  through  being  born,  as  the  off- 
spring of  parents  of  its  kind,  that  we  shall  likely  not  appre- 
ciate at  first  the  full  significance  of  the  statement  that  all 
life  comes  from  life ;  that  all  organisms  are  produced  by 
other  organisms.  Nor  shall  we  at  first  appreciate  the  im- 
portance of  the  statement.  This  is  a  generalization  of 
modern  times.  It  has  always  been  easy  to  see  that  cats 
and  horses  and  chickens  and  the  other  animals  we  famil- 
iarly know  give  birth  to  young  or  new  animals  of  their 
own  kind ;  or,  put  conversely,  that  young  or  new  cats  and 
horses  and  chickens  come  into  existence  only  as  the  off- 
60 


THE  MULTIPLICATION  OF  ANIMALS  AND  SEX       51 

spring  of  parents  of  their  kind.  And  in  these  latter  days  of 
microscopes  and  mechanical  aids  to  observation  it  is  even 
easy  to  see  that  the  smaller  animals,  the  microscopic  organ- 
isms, come  into  existence  only  as  they  are  produced  by  the 
division  of  oijier  similar  animals,  which  we  may  call  their 
parents.  But  in  the  days  of  the  earlier  naturalists  the 
life  of  the  microscopic  organisms,  and  even  that  of  many 
of  the  larger  but  unfamiliar  animals,  was  shrouded  in 
mystery.  And  what  seem  to  us  ridiculous  beliefs  were 
held  regarding  the  origin  of  new  individuals. 

29.  Spontaneous  generation. — The  ancients  believed  that 
many  animals  were  spontaneously  generated.  The  early 
naturalists  thought  that  flies  arose  by  spontaneous  genera- 
tion from  the  decaying  matter  of  dead  animals ;  from  a 
dead  horse  come  myriads  of  maggots  which  change  into 
flesh  flies.  Frogs  and  many  insects  were  thought  to  be 
generated  spontaneously  from  mud.  Eels  were  thought  to 
arise  from  the  slime  rubbed  from  the  skin  of  fishes.  Aris- 
totle, the  Greek  philosopher,  who  was  the  greatest  of  the 
ancient  naturalists,  expresses  these  beliefs  in  his  books.  It 
was  not  until  the  middle  of  the  seventeenth  century- 
Aristotle  lived  three  hundred  and  fifty  years  before  the 
birth  of  Christ— that  these  beliefs  were  attacked  and  be- 
gan to  be  given  up.  In  the  beginning  of  the  seventeenth 
century  William  Harvey,  an  English  naturalist,  declared 
that  every  animal  comes  from  an  egg,  but  he  said  that  the 
egg  might  "  proceed  from  parents  or  arise  spontaneously  or 
out  of  putrefaction."  In  the  middle  of  the  same  century 
Eedi  proved  that  the  maggots  in  decaying  meat  which  pro- 
duce the  flesh  flies  develop  from  eggs  laid  on  the  meat  by 
flies  of  the  same  kind.  Other  zoologists  of  this  time  were 
active  in  investigating  the  origin  of  new  individuals.  And 
all  their  discoveries  tended  to  weaken  the  belief  in  the 
theory  of  spontaneous  generation. 

Finally,  the  adherents  of  this  theory  were  forced  to 
restrict  their  belief  in  spontaneous  generation  to  the  case 


52  ANIMAL  LIFE 

of  a  few  kinds  of  animals,  like  parasites  and  the  animalcules 
of  stagnant  water.  It  was  maintained  that  parasites  arose 
spontaneously  from  the  matter  of  the  living  animal  in 
which  they  lay.  Many  parasites  have  so  complicated  and 
extraordinary  a  life  history  that  it  was  only  after  long  and 
careful  study  that  the  truth  regarding  their  origin  was  dis- 
covered. But  in  the  case  of  every  parasite  whose  life  his- 
tory is  known  the  young  are  offspring  of  parents,  of  other 
individuals  of  their  kind.  No  case  of  spontaneous  genera- 
tion among  parasites  is  known.  The  same  is  true  of  the 
animalcules  of  stagnant  water.  If  some  water  in  which 
there  are  apparently  no  living  organisms,  however  minute, 
be  allowed  to  stand  for  a  few  days,  it  will  come  to  be 
swarming  with  microscopic  plants  and  animals.  Any  or- 
ganic liquid,  as  a  broth  or  a  vegetable  infusion  exposed  for 
a  short  time,  becomes  foul  through  the  presence  of  innumer- 
able bacteria,  infusoria,  and  other  one-celled  animals  and 
plants,  or  rather  through  the  changes  produced  by  their 
life  processes.  But  it  has  been  certainly  proved  that  these 
organisms  are  not  spontaneously  produced  by  the  water  or 
organic  liquid.  A  few  of  them  enter  the  water  from  the 
air,  in  which  there  are  always  greater  or  less  numbers  of 
spores  of  microscopic  organisms.  These  spores  (embryo  or- 
ganisms in  the  resting  stage)  germinate  quickly  when  they 
fall  into  water  or  some  organic  liquid,  and  the  rapid  suc- 
cession of  generations  soon  gives  rise  to  the  hosts  of  bacteria 
and  Protozoa  which  infest  all  standing  water.  If  all  the 
active  organisms  and  inactive  spores  in  a  glass  of  water  are 
killed  by  boiling  the  water,  "  sterilizing  "  it,  as  it  is  called, 
and  this  sterilized  water  or  organic  liquid  be  put  into  a 
sterilized  glass,  and  this  glass  be  so  well  closed  that  germs 
or  spores  can  not  pass  from  the  air  without  into  the  steril- 
ized liquid,  no  living  animals  will  ever  appear  in  it.  It  is 
now  known  that  flesh  will  not  decay  or  liquids  ferment 
except  through  the  presence  of  living  animals  or  plants. 
To  sum  up,  we  may  say  that  we  know  of  no  instance  of  the 


THE  MULTIPLICATION  OF  ANIMALS  AND  SEX       53 

spontaneous  generation  of  organisms,  and  that  all  the  ani- 
mals whose  life  history  we  know  are  produced  from  other 
animals  of  the  same  kind.  "  Omne  vivum  ex  vivo,"  All  life 
from  life. 


FIG.  26.— The  multiplication  of  Amoeba  by  simple  fission. 

30.  The  simplest  method  of  multiplication. — In  our  study 
of  the  simplest  and  the  slightly  complex  animals  we  became 
acquainted  with  the  simplest  methods  of  multiplication 
and  with  methods  which  are  more  complex.  The  method 


54  ANIMAL  LIFE 

of  simple  fission  or  splitting — binary  fission  it  is  often  called, 
because  the  division  is  always  in  two — by  which  the  body 
of  the  parent  becomes  divided  into  two  equal  parts— into 
halves — is  the  simplest  method  of  multiplication.  This  is 
the  usual  method  of  Amoeba  (Fig.  26)  and  of  many  other  of 
the  simplest  animals.  In  this  kind  of  reproduction  it  is 
hardly  exact  to  speak  of  parent  *and  children.  The  chil- 
dren, the  new  Amwbce,  are  simply  the  parent  cut  into 
halves.  The  parent  persists ;  it  does  not  produce  off- 
spring and  die.  Its  whole  body  continues  to  live.  The 
new  Amoeba  take  in  and  assimilate  food  and  add  new  mat- 
ter to  the  original  matter  of  the  parent  body ;  then  each 
of  them  divides  in  two.  The  grandparent's  body  is  now 
divided  into  four  parts,  one  fourth  of  it  forming  one  half 
of  each  of  the  bodies  of  the  four  grandchildren.  The  pro- 
cess of  assimilation,  growth,  and  subsequent  division  takes 
place  again,  and  again,  and  again.  Each  time  there  is  given 
to  the  new  Amoeba  an  ever-lessening  part  of  the  actual 
body  substance  of  the  original  ancestor.  Thus  an  Amceba 
never  dies  a  natural  death,  or,  as  has  been  said,  "no  Amoeba 
ever  lost  an  ancestor  by  death."  It  may  be  killed  outright, 
but  in  that  case  it  leaves  no  descendants.  If  it  is  not  killed 
before  it  produces  new  Amosbce  it  never  dies,  although  it 
ceases  to  exist  as  a  single  individual.  The  Amceba  and 
other  simple  animals  which  multiply  by  direct  binary 
fission  may  be  said  to  be  immortal,  £nd  the  "  immortality 
of  the  Protozoa  "  is  a  phrase  which  you  will  be  sure  to  meet 
if  you  begin  to  read  the  writings  of  the  modern  philosoph- 
ical zoologists. 

31.  Slightly  complex  methods  of  multiplication.— Most  of 
the  Protozoa  multiply  or  reproduce  themselves  in  two 
ways — by  simple  fission  and  by  conjugation.  Paramce- 
cium,  for  example,  reproduces  itself  for  many  generations 
by  fission,  but  a  generation  finally  appears  in  which  a  dif- 
ferent method  of  reproduction  is  followed.  Two  individu- 
als come  together  and  each  exchanges  with  the  other  a  part 


THE  MULTIPLICATION  OF  ANIMALS  AND  SEX       55 

of  its  nucleus.  Then  the  two  individuals  separate  and 
each  divides  into  two.  The  result  of  this  conjugation  is 
to  give  to  the  new  Paramwcia  produced  by  the  conjugat- 
ing individuals  a  body  which  contains  part  of  the  body 
substance  of  two  distinct  individuals.  The  new  Paramoe- 
cia  are  not  simply  halves  of  a  single  parent ;  they  are  parts 
of  two  parents.  If  the  two  conjugating  individuals  differ 
at  all — and  they  always  do  differ,  because  no  two  individual 
animals,  although  belonging  to  the  same  species,  are  exactly 
alike — the  new  individual,  made  up  of  parts  of  each  of  them, 
will  differ  from  both.  "We  shall,  as  we  study  further,  see 
that  Nature  seems  intent  on  making  every  new  individual 
differ  slightly  from  the  individual  which  produces  it ;  and 
the  method  of  multiplication  or  the  production  of  new  indi- 
viduals which  Nature  has  adopted  to  produce  the  result  is 
the  method  which  we  have  seen  exhibited  in  its  simplest 
form  among  the  simplest  animals — the  method  of  having 
two  individuals  take  part  in  the  production  of  a  new  one. 
The  further  study  of  multiplication  among  animals  is  the 
study  of  the  development  and  elaboration  of  this  method. 
32.  Differentiation  of  the  reproductive  cells. — Among  the 
colonial  Protozoa  the  first  differentiation  of  the  cells  or 
members  composing  the  colony  is  the  differentiation  into 
two  kinds  of  reproductive  cells.  Reproduction  by  simple 
division,  without  preceding  conjugation,  can  and  does  take 
place,  to  a  certain  extent,  among  all  the  colonial  Protozoa. 
Indeed,  this  simple  method  of  multiplication,  or  some  modi- 
fication of  it,  like  budding,  persists  among  many  of  the  com- 
plex animals,  as  the  sponges,  the  polyps,  and  even  higher 
and  more  complex  forms.  But  such  a  method  of  single- 
parent  reproduction  can  not  be  used  alone  by  a  species  for 
many  generations,  and  those  animals  which  possess  the 
power  of  multiplication  in  this  way  always  exhibit  also  the 
other  more  complex  kind  of  multiplication,  the  method  of 
double-parent  reproduction.  Conjugation  takes  place  be- 
tween different  members  of  a  single  colony  of  one  of  the 


56  ANIMAL  LIFE 

colonial  Protozoa,  or  between  members  of  different  colonies 
of  the  same  species.  These  conjugating  individuals  in  the 
simpler  kinds  of  colonies,  like  Gonium,  are  similar;  in 
Pandorina  they  appear  to  be  slightly  different,  and  in  Eudo- 
rina  and  Volvox  the  conjugating  cells  are  very  different  from 
each  other  (Figs.  15  and  16).  One  kind  of  cell,  which  is 
called  the  egg  cell,  is  large,  spherical,  and  inactive,  while 
the  other  kind,  the  sperm  cell,  is  small,  with  ovoid  head 
and  tapering  tail,  and  free-swimming.  In  the  simpler  colo- 
nial Protozoa  all  the  cells  of  the  body  take  part  in  repro- 
duction, but  in  Volvox  only  certain  cells  perform  this  func- 
tion, and  the  other  cells  of  the  body  die.  Or  we  may  say 
that  the  body  of  Volvox  dies  after  it  has  produced  special 
reproductive  cells  which  shall  fulfill  the  function  of  multi- 
plication. 

Beginning  with  the  more  complex  Volvocinas,  which  we 
may  call  either  the  most  complex  of  the  one-celled  animals 
or  the  simplest  of  the  many-celled  animals,  all  the  complex 
animals  show  this  distinct  differentiation  between  the  re- 
productive cells  and  the  cells  of  the  rest  of  the  body.  Of 
course,  we  find,  as  soon  as  we  go  up  at  all  far  in  the  scale  of 
the  animal  world,  that  there  is  a  great  deal  of  differentia- 
tion among  the  cells  of  the  body :  the  cells  which  have  to 
do  with  the  assimilation  of  food  are  of  one  kind ;  those  on 
which  depend  the  motions  of  the  body  are  of  another  kind ; 
those  which  take  oxygen  and  those  which  excrete  waste 
matter  are  of  other  kinds.  But  the  first  of  this  cell  differ- 
entiation, as  we  have  already  often  repeated,  is  that  shown 
by  the  reproductive  cells ;  and  with  the  very  first  of  this 
differentiation  between  reproductive  cells  and  the  other 
body  cells  appears  a  differentiation  of  the  reproductive 
cells  into  two  kinds.  These  two  kinds,  among  all  animals, 
are  always  essentially  similar  to  the  two  kinds  shown  by 
Volvox  and  the  simplest  of  the  many-celled  animals — namely, 
large,  inactive,  spherical  egg  cells,  and  small,  active,  elon- 
gate or  "  tailed  "  sperm  cells. 


THE  MULTIPLICATION  OF   ANIMALS  AND  SEX       57 

33.  Sex,  or  male  and  female. — In  the  slightly  complex 
animals  one  individual  produces  both  egg  cells  and  sperm 
cells.    But  in  the  Siphonophora,  or  colonial  jelly-fishes,  stud- 
ied in  the  last  chapter,  certain  members  of  the  colony  pro- 
duce only  sperm  cells,  and  certain  other  members  of  the 
colony  produce  only  egg  cells.     If  the  Siphonophora  be 
considered  an  individual  organism  and  not  a  colony  com- 
posed of  many  individuals,  then,  of  course,  it  is  like  the 
others  of  the  slightly  complex  animals  in  this  respect.     But 
as  soon  as  we  rise  higher  in  the  scale  of  animal  life,  as  soon 
as  we  study  the  more  complex  animals,  we  find  that  the 
egg  cells  and  sperm  cells  are  almost  always  produced  by 
different  individuals.      Those    individuals  which  produce 
egg  cells  are  called  female,  and  those  which  produce  sperm 
cells  are  called  male.      There   are  two  sexes.     Male  and 
female  are  terms  usually  applied  only  to  individuals,  but 
it  is  evidently  fair  to  call  the  egg  cells  the  female  reproduc- 
tive cells,  and  the  sperm  cells  the  male  reproductive  cells. 
A  single  individual  of  the  simpler  kinds  of  animals  pro- 
duces both  male  and  female  cells.     But  such  an  individual 
can  not  be  said  to  be  either  male  or  female ;  it  is  sexless — 
that  is,  sex  is  something  which  appears  only  after  a  certain 
degree  of   structural   and   physiological  differentiation  is 
reached.     It  is  true  that  even  among  many  of  the  higher 
or  complex  animals  certain  species  are  not  represented  by 
male  and  female  individuals,  any  individual  of  the  species 
being  able  to  produce  both  male  and  female  cells.    But  this 
is  the  exception. 

34.  The  object  of  sex. — Among  almost  all  the  complex 
animals  it  is  necessary  that  there  be  a  conjugation  of  male 
and  female  reproductive  cells  in  order  that  a  new  individual 
may  be  produced.     This  necessity  first  appears,  we  remem- 
ber, among  very  simple  animals.     This  intermixing  of  body 
substance  from  two  distinct  individuals,  and  the  develop- 
ment therefrom  of  the  new  individual,  is  a  phenomenon 
which  takes  place  through  the  whole  scale  of  animal  life. 


58  ANIMAL  LIFE 

The  object  of  this  intermixing  is  the  production  of  va- 
riation. Nature  demands  that  the  offspring  shall  differ 
slightly  from  its  parents.  By  having  the  beginnings  of  its 
body,  the  single  cell  from  which  the  whole  body  develops, 
composed  of  parts  of  two  different  individuals,  this  differ- 
ence, although  slight  and  nearly  imperceptible,  is  insured. 
Sex  is  a  provision  of  Nature  which  insures  variation. 

35.  Sex  dimorphism. — As  we  have  seen,  almost  every 
species  of  animal  is  represented  by  two  kinds  of  individuals, 
males  and  females.  In  the  case  of  many  animals,  espe- 


FIG.  27. — Bird  of  paradise,  male. 


cially  the  simpler  ones,  these  two  kinds  of  individuals  do 
not  differ  in  appearance  or  in  structure  apart  from  the 
organs  concerned  with  multiplication.  But  with  many 
animals  the  sexes  can  be  readily  distinguished.  The  male 
and  female  individuals  often  show  marked  differences, 
especially  in  external  structural  characters.  We  can  read- 


THE  MULTIPLICATION  OF  ANIMALS  AND  SEX       59 

ily  tell  the  peacock,  with  its  splendidly  ornamental  tail 
feathers,  from  the  unadorned  peafowl,  or  the  horned  ram 
from  the  bleating  ewe.  There  is  here,  plainly,  a  dimor- 
phism— the  existence  of  two  kinds  of  individuals  belonging 
to  a  single  species.  This  dimorphism  is  due  to  sex,  and 
the  condition  may  be  called  sex  dimorphism.  Among  some 
animals  this  sex  dimorphism,  or  difference  between  the 
sexes,  is  carried  to  extraordinary  extremes.  This  is  espe- 
cially true  among  polygamous  animals,  or  those  in  which 
the  males  mate  with  many  females,  and  are  forced  to  fight 
for  their  possession.  The  male  bird  of  paradise,  with  its 
gorgeous  display  of  brilliantly  colored  and  fantastically 
shaped  feathers  (Fig.  27),  seems  a  wholly  different  kind  of 
bird  from  the  modest  brown  female.  The  male  golden  and 
silver  pheasants,  and  allied  species  with  their  elaborate 
plumage,  are  very  unlike  the  dull-colored  females.  The 
great,  rough,  warlike  male  fur  seal,  roaring  like  a  lion,  is 
three  times  as  large  as  the  dainty,  soft-furred  female,  which 
bleats  like  a  sheep. 

Among  some  of  the  lower  animals  the  differences  be- 
tween male  and  female  are  even  greater.  The  males  of 
the  common  cankerworm  moth  (Fig.  28)  have  four  wings ; 


FIG.  28.— Cankerworm  moth ;  the  winged  male  and  wingless  female. 

the  females  are  wingless,  and  several  other  insect  species 
show  this  same  difference.  Among  certain  species  of  white 
ants  the  females  grow  to  be  five  or  six  inches  long,  while 
the  males  do  not  exceed  half  an  inch  in  length.  In  the 


60 


ANIMAL  LIFE 


case  of  some  of  the  parasitic  worms  which  live  in  the  bod- 
ies of  other  animals,  the  male  has  an  extraordinarily  de- 
graded, simple  body,  much  smaller  than  that  of  the  female 
and  differing  greatly  from  that  of  the  female  in  structure. 
In  some  cases  even — as,  for  example, 
the  worm  which  causes  "  gapes  "  in 
chickens  —  the  male  lives  parasiti- 
cally  on  the  female,  being  attached  to 
the  body  of  the  female  for  its  whole 
lifetime,  and  drawing  its  nourish- 
ment from  her  blood  (Fig.  29). 

A  condition  known  as  partheno- 
genesis is  found  among  certain  of 
the  complex  animals.  Although  the 
species  is  represented  by  individu- 
als of  both  sexes,  the  female  can 
produce  young  from  eggs  which 
have  not  been  fertilized.  For  ex- 
ample, the  queen  bee  lays  both  fer- 
tilized and  unfertilized  eggs.  From 
the  fertilized  eggs  hatch  the  work- 
ers, which  are  rudimentary  females, 
and  other  queens,  which  are  fully- 
FIG.  29.-The  parasitic  worm  Developed  females ;  from  the  unfer- 

(Syngamus  trachealis),  which  A  _ 

causes  the  "  gapes "  in  fowls,   tilized  eggs  hatch  only  males — the 
The  male  is  attached  to  the  drones.     Many  generations  of  plant 

female,  and  lives  as  a  para-    , . 

site  on  her.  lice  are  produced  each  year  parthe- 

nogenetically  —  that  is,  by  unferti- 
lized females.  But  there  is  at  least  one  generation  each 
year  produced  in  the  normal  way  from  fertilized  eggs. 

Some  of  the  complex  animals  are  hermaphroditic — that 
is,  a  single  individual  produces  both  egg  cells  and  sperm 
cells.  The  tapeworm  and  many  allied  worms  show  this 
condition.  This  is  the  normal  condition  for  the  simplest 
animals,  as  we  have  already  learned,  but  it  is  an  excep- 
tional condition  among  the  complex  animals. 


THE  MULTIPLICATION  OF  ANIMALS  AND  SEX       61 

36.  The  number  of  young. — There  is  great  variation  in  the 
number  of  young  produced  by  different  species  of  animals. 
Among  the  animals  we  know  familiarly,  as  the  mammals, 
which  give  birth  to  young  alive,  and  the  birds,  which  lay 
eggs,  it  is  the  general  rule  that  but  few  young  are  pro- 
duced at  a  time,  and  the  young  are  born  or  eggs  are  laid 
only  once  or  perhaps  a  few  times  in  a  year.  The  robin  lays 
five  or  six  eggs  once  or  twice  a  year ;  a  cow  may  produce 
a  calf  each  year.  Rabbits  and  pigeons  are  more  prolific, 
each  having  several  broods  a  year.  But  when  we  observe 
the  multiplication  of  some  of  the  animals  whose  habits  are 
not  so  familiar  to  us,  we  find  that  the  production  of  so  few 
young  is  the  exceptional  and  not  the  usual  habit.  A  lob- 
ster lays  ten  thousand  eggs  at  a  time ;  a  queen  bee  lays 
about  five  million  eggs  in  her  life  of  four  or  five  years.  A 
female  white  ant,  which  after  it  is  full  grown  does  nothing 
but  lie  in  a  cell  and  lay  eggs,  produces  eighty  thousand 
eggs  a  day  steadily  for  several  months.  A  large  codfish 
was  found  on  dissection  to  contain  about  eight  million 


If  we  search  for  some  reason  for  this  great  difference  in 
fertility  among  different  animals,  we  may  find  a  promis- 
ing clew  by  attending  to  the  duration  of  life  of  animals, 
and  to  the  amount  of  care  for  the  young  exercised  by  the 
parents.  We  find  it  to  be  the  general  rule  that  animals 
which  live  many  years,  and  which  take  care  of  their  young, 
produce  but  few  young ;  while  animals  which  live  but  a 
short  time,  and  which  do  not  care  for  their  young,  are  very 
prolific.  The  codfish  produces  its  millions  of  eggs ;  thou- 
sands are  eaten  by  sculpins  and  other  predatory  fishes  be- 
fore they  are  hatched,  and  other  thousands  of  the  defense- 
less young  fish  are  eaten  long  before  attaining  maturity. 
Of  the  great  number  produced  by  the  parent,  a  few  only 
reach  maturity  and  produce  new  young.  But  the  eggs  of  the 
robin  are  hatched  and  protected,  and  the  helpless  fledglings 
are  fed  and  cared  for  until  able  to  cope  with  their  natural 


62  ANIMAL  LIFE 

•» 

enemies.     In  the  next  year  another  brood  is  carefully  reared, 
and  so  on  for  the  few  years  of  the  robin's  life. 

Under  normal  conditions  in  any  given  locality  the  num- 
ber of  individuals  of  a  certain  species  of  animal  remains 
about  the  same.  The  fish  which  produces  tens  of  thousands 
of  eggs  and  the  bird  which  reproduces  half  a  dozen  eggs  a 
year  maintain  equally  well  their  numbers.  In  one  case  a 
few  survive  of  many  born ;  in  the  other  many  (relatively) 
survive  of  the  few  born  ;  in  both  cases  the  species  is  effect- 
ively maintained.  In  general,  no  agency  for  the  perpetua- 
tion of  the  species  is  so  effective  as  that  of  care  for  the 
young. 


CHAPTEE  IV 

FUNCTION   AND   STRUCTURE 

37.  Organs  and  functions.— An  animal  does  certain  things 
which  are  necessary  to  life.  It  eats  and  digests  food,  it 
breathes  in  air  and  takes  oxygen  from  it  and  breathes  out 
carbonic-acid  gas ;  it  feels  and  has  other  sensations ;  it  pro- 
duces offspring,  thus  reproducing  itself.  These  things  are 
done  by  the  simplest  animals  as  well  as  by  the  complex 
animals.  But  while  with  the  simplest  animals  the  whole 
body  (which  is  but  a  single  cell)  takes  part  in  doing  each 
of  these  things,  among  the  complex  animals  only  a  part 
of  the  body  is  concerned  with  any  one  of  these  things. 
Only  a  part  of  the  body  has  to  do  with  the  taking  in  of 
oxygen.  Another  part  has  to  do  with  the  digestion  of 
food,  and  another  with  the  business  of  locomotion.  These 
parts  of  the  body,  as  we  know,  differ  from  each  other,  and 
they  differ  because  they  have  different  things  to  do.  These 
different  parts  are  called  organs  of  the  body,  and  the  things 
they  do  are  called  their  functions.  The  nostrils,  tracheae, 
and  lungs  are  the  organs  which  have  for  function  the  pro- 
cess of  respiration.  The  legs  of  a  cat  are  the  organs  which 
perform  for  it  the  function  of  locomotion.  The  structure 
of  one  of  the  higher  animals  is  complex  because  the  body 
is  made  up  of  many  distinct  organs  having  distinct  func- 
tions. The  things  done  by  one  of  the  complex  animals  are 
many ;  around  each  of  the  principal  functions  or  necessary 
processes,  as  a  center,  are  grouped  many  minor  accessory 
functions,  all  helping  to  make  more  successful  the  accom- 

63 


64  ANIMAL  LIFE 

plishment  of  the  principal  functions.  "While  many  of  the 
lower  animals  have  no  eyes  and  no  ears,  and  trust  to  more 
primitive  means  to  discover  food  or  avoid  enemies,  the 
higher  animals  have  extraordinarily  complex  organs  for 
seeing  and  hearing,  two  functions  which  are  accessory  only 
to  such  a  principal  function  as  food-taking. 

38.  Differentiation  of  structure, — We  have  seen,  in  our 
study  of  the  slightly  complex  animals,  how  the  body  be- 
comes more  and  more  complex  in  proportion  to  the  degree 
in  which  the  different  life  processes  are  divided  or  assigned 
to  different  parts  of  it  for  performance.     With  the  gradu- 
ally increasing  division  of  labor  the   body  becomes  less 
homogeneous  in  structure;  a  differentiation  of  structure 
becomes  apparent  and  gradually  increases.     The  extent  of 
the  division  of  labor  and  the  extent  of  the  differentiation 
of  structure,  or  division  of  the  body  into  distinct  and  dif- 
ferent parts  and  organs,  go  hand  in  hand.     An  animal  in 
which  the  division  of  labor  is  carried  to  an  extreme  is  an 
animal  in  which  complexity  of  structure  is  extreme. 

39.  Anatomy  and  physiology. — Zoology,  or  the  study  of 
animals,  is  divided  for  convenience  into  several  branches 
or  phases.     The  study  of  the  classification  of  animals  is 
called  systematic  zoology;  the  study  of  the  development 
of  animals  from  their  beginning  as  a  single  cell  to  the  time 
of  their  birth  is  called  animal  embryology ;  the  study  of 
the  structure  of  animals  is  called  animal  anatomy,  and  the 
study  of  the  performance  of  their  life  processes  or  functions 
is  called  physiology.     Because  the  whole  field  of  zoology  is 
so  great,  some  zoologists  limit  themselves  exclusively  to  one 
of  these  phases  of  zoological  study,  and  those  who  do  not 
so  definitely  limit  their  study,  at  least  give  their  special  at- 
tention to  a  single  phase,  although  all  try  to  keep  in  touch 
with  the  state  of  knowledge  in  other  phases.     In  earlier 
days  the  study  of  the  anatomy  of  animals  and  of  their 
physiology  were  held  to  be  two  very  distinct  lines  of  in- 
vestigation, and  the  anatomists  paid  little  attention  to 


FUNCTION  AND  STRUCTURE  65 

physiology  and  the  physiologists  little  to  anatomy.  But 
we  have  seen  how  inseparably  linked  are  structure  and 
function.  The  structure  of  an  animal  is  as  it  is  because 
of  the  work  it  has  to  do,  and  the  functions  of  an  animal 
are  performed  as  they  are  performed  because  of  the  special 
structural  condition  of  the  organs  which  perform  them. 
The  study  of  the  anatomy  and  the  study  of  the  physiology 
of  animals  can  not  be  separated.  To  understand  aright 
the  structure  of  an  animal  it  is  necessary  to  know  to 
what  use  the  structure  is  put ;  to  understand  aright  the 
processes  of  an  animal  it  is  necessary  to  know  the  struc- 
ture on  which  the  performance  of  the  processes  depends. 

40.  The  animal  body  a  machine. — The  body  of  an  animal 
may  be  well  compared  with  some  machine  like  a  locomotive 
engine.  Indeed,  the  animal  body  is  a  machine.  It  is  a 
machine  composed  of  many  parts,  each  part  doing  some 
particular  kind  of  work  for  which  a  particular  kind  of 
structure  fits  it ;  and  all  the  parts  are  dependent  on  each 
other  and  work  together  for  the  accomplishment  of  the 
total  business  of  the  machine.  The  locomotive  must  be 
provided  with  fuel,  such  as  coal  or  wood  or  other  readily 
combustible  substance,  the  consumption  of  which  furnishes 
the  force  or  energy  of  the  machine.  The  animal  body 
must  be  provided  with  fuel,  which  is  called  food,  which 
furnishes  similarly  the  energy  of  the  animal.  Oxygen  must 
be  provided  for  the  combustion  of  the  fuel  in  the  locomo- 
tive and  the  food  in  the  body.  The  locomotive  is  com- 
posed of  special  parts  :  the  firebox  for  the  reception  and 
combustion  of  fuel;  the  steam  pipes  for  the  carriage  of 
steam  ;  the  wheels  for  locomotion  ;  the  smoke  stack  for 
throwing  off  of  waste.  The  animal  body  is  similarly  com- 
posed of  parts  :  the  alimentary  canal  for  the  reception  and 
assimilation  of  food  ;  the  excretory  organs  for  the  throwing 
off  of  waste  matter  ;  the  arteries  and  veins  for  the  carriage 
of  the  oxygen  and  food-holding  blood ;  the  legs  or  wings 
for  locomotion. 
6 


66  ANIMAL  LIFE 

The  locomotive  is  an  inorganic  machine ;  the  animal  is 
an  organic  machine.  There  is  a  great  and  real  difference 
between  an  organism,  a  living  animal,  and  a  locomotive,  an 
inorganic  structure.  But  for  a  good  understanding  of  the 
relation  between  function  and  structure,  and  of  the  com- 
position of  the  body  of  the  complex  animals,  the  compari- 
son of  the  animal  and  locomotive  is  very  instructive. 

41.  The  specialization  of  organs.— The  organ  for  the  per- 
formance of  some  definite  function  in  one  of  the  higher 
animals  may  be  very  complex.     The  corresponding  organ 
in  one  of  the  lower  animals  for  the  performance  of  the 
same  function  may  be  comparatively  simple.     For  example, 
the  organ  for  the  digestion  of  food  is,  in  the  case  of  the 
polyp,  a  simple  cylindrical  cavity  in  the  body  into  which 
food  enters  through  a  large  opening  at  the  apical  or  free 
end  of  the  body.     The  digestive  organ  of  a  cow  is  a  long 
coiled  tube,  comprising  many  regions  of  distinct  structural 
and  physiological  character  and  altogether  extremely  com- 
plicated.    An  organ  in  simple  or  primitive  condition  is 
said  to  be  generalized ;  in  complex  or  highly  modified  con- 
dition it  is  said  to  be  specialized.     That  is,  an  organ  may 
be  modified  and  complexly  developed  to  perform  its  func- 
tion in  a  special  way,  in  a  way  differing  in  many  particu- 
lars from  the  way  the  corresponding  organ  in  some  other 
animal  performs  the  same  general  function.     The  speciali- 
zation of  organs,  or  their  modification  to  perform  their 
functions  in  special  ways,  is  what  makes  animal  bodies 
complex,  for  specialization  is  almost  always  in  the  line  of 
complexity.     Later  we  shall  see  more  clearly  how  specializa- 
tion is  brought  about.      For  the  present  we   may  study 
one  of  the  more  important  organs  of  the  animal  body  for 
the  sake  of  having  concrete  examples  of  some  of  the  gen- 
eral statements  made  in  this  discussion  of  function  and 
structure. 

42.  The  alimentary  canal — The  organ  which  has  to  do 
with  the  taking  and  digesting  of  food  is  called  the  ali- 


FUNCTION  AND  STRUCTURE 


mentary  canal.  In  some  of  the  higher  animals  this  is  a 
very  complex  organ.  In  the  cow,  one  of  the  cud-chewing 
mammals  or  ruminants,  it  consists  of  several  distinct  por- 
tions, which  differ  among  themselves  very  much  (Fig.  30). 
First,  there  is  the  mouth,  or  opening  for  the  entrance  of 
the  food.  The  mouth  is  sup- 
plied with  teeth  for  tearing 
off  and  chewing  the  food, 
with  a  tongue  for  manipu- 
lating it,  and  with  taste  pa- 
pillae situated  on  the  tongue 
and  palate  for  determining 
the  desirability  of  the  food. 
Into  the  mouth  a  peculiar 
fluid  (the  saliva)  is  poured 
by  certain  glands,  organs  ac- 
cessory to  the  alimentary 
canal.  The  herbage  bitten 
off,  mixed  with  saliva,  and 
rolled  by  the  tongue  into  a 
ball,  passes  back  through  a 
narrow  tube,  the  oesophagus, 
and  into  a  sac  called  the  ru- 
men, or  paunch.  Here  it 
lies  until  the  cow  ceases  for 
the  while  to  take  in  food, 
when  it  passes  back  again 
through  the  oesophagus  and 
into  the  mouth  for  mastica- 
tion. After  being  masticated  it  again  passes  downward 
through  the  oesophagus,  and  enters  this  time  another  sac 
called  the  reticulum,  lying  next  to  the  rumen.  From  here 
it  passes  into  another  sac-like  portion  of  the  alimentary 
canal  called  the  omasum,,  where  it  is  strained  through 
numerous  leaf-like  folds  which  line  the  walls  of  this  part 
of  the  canal.  From  here  the  food  passes  into  a  fourth 


t.... 


FIG.  30.— Alimentary  canal  of  the  ox 
(after  COLIN  and  MULLER).  a,  rumen 
(left  hemsiphere)  ;  b,  rumen  (right  hem- 
isphere) ;  c,  insertion  of  oesophagus  ;  d, 
reticulum  ;  e,  omasum  ;  /,  abomasum  ; 
g,  duodenum ;  h  and  i,  jejunum  and 
ileum ;  j,  caecum  ;  1c,  colon,  with  its 
various  convolutions  ;  I,  rectum. 


68 


ANIMAL  LIFE 


sac-like  part  of  the  canal,  called  the  abomasum.  Here 
the  process  of  digestion  goes  on.  The  four  sacs— rumen, 
reticulum,  omasum,  and  abomasum — are  called  stomachs, 
or  they  may  be  considered  to  be  four  chambers  forming 
one  large  stomach.  In  the  abomasum,  or  digesting  stom- 
ach, digestive  fluids  are  poured  from  glands  lining  its 
walls,  and  the  food  becomes  converted  into  a  liquid  called 
chyle.  The  chyle  passes  from  the  stomach  into  a  long, 
narrow,  tubular  portion  of  the  canal  called  the  intestine. 
The  intestine  is  very  long,  and  lies  coiled  in  a  large  mass 
in  the  body  of  the  cow.  The  intestine  is  divided  into 
distinct  regions,  which  vary  in  size  and  in  the  character 
of  the  inner  wall.  These  parts  of  the  intestine  have 
names,  as  duodenum,  jejunum,  ileum,  caecum,  colon,  etc. 
Part  of  the  intestine  is  lined  inside  with  fine  papillae, 
which  take  up  the  chyle  (the  digested  food)  and  pass  it 
through  the  walls  of  the  intestine  to  other  special  organs, 
which  pass  it  on  to  the  blood,  with  which  it  becomes  mixed 
and  carried  by  an  elaborate  system  of  tubes  to  all  parts  of 
the  body.  Part  of  the  grass  taken  into  the  alimentary 
canal  by  the  cow  can  not  be  digested,  and  must  be  got  rid 
of.  This  passes  on  into  a  final  posterior  part  of  the  intes- 
tine called  the  rectum,  and  leaves  the  body  through  the 
anus  or  posterior  opening  of  the  alimentary  canal.  The 
whole  canal  is  more  than  twenty  times  as  long  as  the  body 
of  the  cow ;  it  is  composed  of  parts  of  different  shape  ;  its 
walls  are  supplied  with  muscles  andblooi-vessels  ;  the  inner 
lining  is  covered  with  folds,  papillae,  and  gland  cells.  It  is 
altogether  a  highly  specialized  organ,  a  structurally  com- 
plex and  elaborately  functioning  organ. 

Let  us  now  examine  the  alimentary  canal,  or  organ  of 
digestion,  in  some  of  the  simpler  animals. 

The  Protozoa,  or  simplest  animals,  have  no  special  organ 
at  all.  When  the  surface  of  the  body  of  an  Amoeba,  comes 
into  contact  with  an  organic  particle  which  will  serve  as 
food,  the  surface  becomes  bent  in  at  the  point  of  its  con- 


FUNCTION  AND  STRUCTURE 


69 


tact  with  the  food  particle,  and  the  body  substance  simply 
incloses  the  food  (Fig.  3).  Food  is  taken  in  by  the  sur- 
face. The  whole  outer  surface  of  the  body  is  the  food- 
taking  organ.  In  the  simplest  many-celled  animals,  the 
sponges,  there  is  no  special  food-taking  and  digestive  organ. 
Each  of  the  cells  of  the  body  takes  in  and  assimilates  food 
for  itself.  The  sponge  is  like  a  great  group  of  Amcebce 
holding  fast  to  each  other,  but  each  looking  out  for  its  own 
necessities.  Among  the  m 

polyps,  however,  there 
is  a  definite  organ  of 
digestion — that  is,  food 
is  only  taken  and  di- 
gested by  certain  parts 
of  the  body.  The  sim- 
ple polyp's  body  (Fig.  ^  ^  i 
31)  is  a  cylinder  or  vase 
closed  at  one  end  and 
open  at  the  other  end, 
and  attached  by  the 
closed  end  to  a  rock. 
The  opening  is  usually 
of  less  diameter  than 
the  diameter  of  the 
body,  and  it  is  sur- 
rounded by  a  number 
of  tentacles,  whose 
function  it  is  to  seize  the  food  and  convey  it  to  the  mouth 
opening.  There  are,  of  course,  no  teeth,  no  tongue,  none 
of  the  various  parts  which  are  in  or  are  part  of  the  mouth 
of  the  higher  animals.  The  polyp's  mouth  is  simply  a 
hole  or  opening  into  the  inside  of  the  body.  This  body 
cavity,  or  simplest  of  all  stomachs,  is  simply  the  cylindrical 
or  vase-shaped  hollow  space  inclosed  by  the  body  wall. 
This  space  extends  also  into  the  tentacles.  There  is  no 
other  opening,  no  posterior  or  anal  opening.  We  can  not 


FIG.  31. — Obeliasp.,a  simple  polyp;  vertical  sec- 
tion, highly  magnified,  m,  mouth  opening ; 
al.  s.,  alimentary  sac.  — After  PARKER  and 
HASWELL. 


70 


ANIMAL  LIFE 


speak  of  an  oesophagus  or  intestine  in  connection  with  this 
most  primitive  of  alimentary  sacs.  The  cells  which  line 
the  sacs  show  some  differentiation ;  some  are  gland  cells 
and  secrete  digestive  fluids ;  some  are  amoeboid  and  are 
provided  with  pseudopods  or  flagella  for  seizing  bits  of 
food.  The  food  caught  by  the  tentacles  comes  into  the  ali- 
mentary sac  through  the  opening  or  primitive  mouth,  and 


PIG.  32.— Diagrammatic  sketch  of  a  flat- 
worm  (Planaria),  showing  the 
branched  alimentary  canal,  al.  c.~ 
After  JIJIMA  and  HATSCHEK. 


PIG.  33.— Sea-cucumber  (Holothurian) 
dissected  to  show  alimentary  canal, 
al.  c.— After  LEUCKART. 


what  of  it  is  digestible  is,  by  the  aid  of  the  gland  cells  and 
the  amoeboid  cells,  taken  up  and  assimilated,  while  the  rest 
of  it  is  carried  out  by  water  currents  again  through  the 
single  opening. 

In  the  flat  worms  (Fig.  32)  like  Planaria  (small,  thin, 
flattened  worms  to  be  found  in  the  mud  at  the  bottom  of 
fresh-water  ponds)  the  mouth  opens  into  a  short,  narrow 
tube  which  may  be  called  an  oesophagus.  The  oesophagus 


FUNCTION  AND  STRUCTURE 


71 


connects  the  mouth  with  the  rest  of  the  alimentary  canal, 
which  gives  out  many  side  branches  or  diverticula,  which 
are  themselves  branched,  so  that  the 
alimentary  sac  or  stomach  is  a  system 
of  ramifying  tubes  extending  from  a 
central  main  tube  to  all  parts  of  the 
body   of  the   worm.       There    is    no 
anal  opening.    In  the  round  or  thread 
worms,  of  which  the  deadly  Trichina 
is  an  example,  the  alimentary  canal 

is  a  simple  straight  tube  with  both  \&L  G, 

anterior  or  mouth  opening  and  pos- 
terior or  anal  opening.  In  the  sea- 
urchins  and  sea-cucumbers  (Fig.  33) 
the  alimentary  canal  is  a  simple  tube 
with  two  openings,  but  it  is  longer 
than  the  body  between  mouth  and 
anus,  and  so  is  more  or  less  bent  or 
coiled.  In  the  earthworm  the  ali- 
mentary canal  (Fig.  34),  although  a 
simple  straight  tube  running  through 
the  body,  plainly  shows  a  differentia- 
tion into  particular  regions.  Behind 
the  mouth  opening  the  alimentary 
tube  is  large  and  thick  -  walled  and 
is  called  the  pharynx;  behind  the 
pharynx  it  is  narrower  and  is  called 
the  oesophagus.  Behind  the  oesopha- 
gus it  expands  to  form  a  rounded, 
thin-walled  chamber  called  the  crop, 
and  just  behind  this  there  is  another 
rounded  but  very  thick-walled  cham-  FIG.  34.-Earthworm  diBsected 

.  -i          T-I  J.T  to  show  alimentary  canal, 

ber   called   the   gizzard.      From  the      ^  c 

gizzard  back  the  alimentary  canal  is 

about  uniform  in  size,  being  rather  wide  and  having  thick, 

soft  walls.     This  portion  of  it  is  called  the  intestine.     The 


72  ANIMAL  LIFE 

posterior  part  of  the  intestine,  called  the  rectum,  leads  to 
the  anal  opening.  There  is  some  differentiation  of  the 
inner  surface  of  the  canal.  In  the  great  group  of  mol- 
lusks,  of  which  the  common  fresh-water  clam  or  mussel  is 
an  example,  the  alimentary  canal  (Fig.  35)  shows  much 
variation.  The  microscopic  plants,  which  are  the  food  of 
the  mussel,  are  taken  in  through  the  mouth  and  pass  into 
a  short  oesophagus,  thence  into  a  wide  stomach  and  there 
digested.  Behind  the  stomach  is  a  long,  much-folded,  nar- 
row intestine  which  winds  about  through  the  fleshy  "  foot " 
and  finally  reaches  the  surface  of  the  body,  and  has  an 
anal  opening  at  a  point  opposite  the  position  of  the  mouth. 
Among  the  insects  there  is  a  great  range  in  degree  of 
complexity  of  the  alimentary  canal.  The  digestive  organs 
are,  however,  in  most  insects  in  a  condition  of  high  speciali- 
zation. The  mouth  opening  is  provided  with  well-developed 


Cll  C 


FIG.  35.— Pond  mussel  dissected  to  show  alimentary  canal,  al.  c.— After  HATSCHEK 

and  CORI. 

biting  and  masticating  or  piercing  and  sucking  mouth  parts ; 
pharynx,  oesophagus,  stomach,  and  intestine  are  always  dif- 
ferentiated and  sometimes  greatly  modified.  In  the  com- 
mon cockroach,  for  example  (Fig.  36),  the  mouth  has  a 
complicated  food-getting  apparatus,  and  the  canal,  which 


FUNCTION  AND  STRUCTURE 


73 


is  much  longer  than  the  body  of  the  insect,  and  hence 

much  bent  and  coiled,  consists  of  a  pharynx,  oesophagus, 

fore-stomach  or  proventriculus, 

true  digesting  stomach  or  ven- 

triculus,  intestine,  and  rectum 

which  opens  at  the  posterior 

tip  of  the  body.      The  inner 

lining  of  the  canal  shows  much 

differentiation  in  the  different 

parts  of  the  canal,  and  there 

are  numerous  accessory  glands 

connected  with  various  parts  of 

the  canal. 

Finally,  among  the  highest 
animals,  the  vertebrates,  we 
find  still  more  elaborate  special- 
ization of  the  alimentary  canal. 
As  an  example  the  alimentary 
canal  of  a  cow  has  already  been 
described  in  detail. 

43.  Stable  and  variable  char- 
acteristics   of    an    organ.  —  In 
spite  of  all  this  variation   in 

the  structure  and  general  character  of  the  alimentary 
canal,  there  are  certain  characteristics  which  are  features 
of  all  alimentary  canals.  In  the  examination  of  an  organ 
we  must  ever  distinguish  between  its  so-called  constant  or 
stable  characteristics  and  its  inconstant  or  variable  charac- 
teristics. The  constant  characteristics  are  the  fundamen- 
tally essential  ones  of  the  organ ;  the  variable  ones  are  the 
special  characteristics  which  adapt  the  organ  for  the  pecul- 
iar habits  of  the  animal  possessing  it — habits  which  may 
differ  very  much  from  those  of  some  other  animal  of  similar 
size,  similar  distribution,  similar  abundance. 

44.  Stable  and  variable  characteristics  of  the  alimentary 
canal. — A  tiger  or  a  lion  has  an  alimentary  canal  not  more 


PIG.  36.— Cockroach  dissected  to  show 
alimentary  canal,  al.  c.— After  HAT- 
SCHEK  and  Com. 


74  ANIMAL  LIFE 

than  three  or  four  times  the  length  of  its  body,  while  a 
sheep  has  an  alimentary  canal  twenty-eight  times  as  long 
as  its  body.  The  tiger  is  carnivorous ;  the  sheep  her- 
bivorous. Associated  with  the  different  food  habits  of  the 
two  animals  is  a  striking  difference  in  the  alimentary 
canals.  Animals  like  the  horse  or  cat,  which  chew  their 
food  before  swallowing  it,  have  a  slender  oesophagus ;  ani- 
mals like  snakes  which  swallow  their  food  whole  have  a 
wide  oesophagus.  Birds,  that  have  no  teeth  and  hence 
can  not  masticate  or  grind  their  food  in  their  mouths,  usu- 
ally have  a  special  grinding  stomach,  the  gizzard,  for  this 
purpose.  And  so  we  might  cite  innumerable  examples 
of  these  inconstant  or  variable  characteristics  of  the  ali- 
mentary canal.  On  the  other  hand,  the  alimentary  canals 
of  all  the  many-celled  animals  except  the  lowest  agree  in 
certain  important  characteristics.  Each  alimentary  canal 
has  two  openings,  one  for  the  ingress  of  food  and  one  for 
the  exit  of  the  indigestible  portions  of  the  matter  taken  in, 
and  the  canal  itself  stretches  through  the  body  from  mouth 
to  anus  as  a  tube,  now  narrow,  now  wide,  now  suddenly 
expanding  into  a  sac  or  giving  off  lateral  diverticula,  but 
always  simply  a  lumen  or  hollow  inclosed  by  a  flexible  mus- 
cular wall.  The  inner  lining  of  the  wall  is  provided  with 
secreting  and  absorbing  structures.  Indeed,  we  can  reduce 
the  essential  characters  of  the  alimentary  canal  to  even 
more  simple  features.  The  organ  of  digestion  or  assimila- 
tion of  all  the  many-celled  animals  is  merely  a  surface  with 
which  food  is  brought  into  contact,  and  which  has  the 
power  of  digesting  this  food  by  means  of  digestive  secre- 
{  tions,  and  of  absorbing  the  food  when  digested.  This  sur- 
face is  small  or  great  in  extent,  depending  upon  the  amount 
of  food  necessary  to  the  life  of  the  animal  and  the  difficulty 
or  readiness  with  which  the  food  can  be  digested.  This 
surface  might  just  as  well  be  on  the  outside  of  the  animal's 
body  as  on  the  inside,  if  it  were  convenient.  In  fact,  it  is 
on  the  outside  of  some  animals.  Among  the  Protozoa  the 


FUNCTION  AND  STRUCTURE  75 

digesting  surface  is  simply  the  external  surface  of  the  body. 
And  not  alone  among  the  one-celled  animals.  Many  of  the 
parasitic  worms  which  live  in  the  bodies  of  other  animals, 
and  the  larvae  or  "  grubs  "  of  many  insects  which  lie  in  the 
tissues  of  plants  bathed  by  the  sap,  have  no  inner  alimen- 
tary canal,  but  take  food  through  the  outer  surface  of  the 
body.  But  in  these  cases  the  food  is  ready  for  immediate 
absorption,  so  that  no  special  treatment  of  it  is  necessary, 
hence  no  complex  structures  are  required. 

Even  were  no  such  special  treatment  of  the  food  neces- 
sary in  the  case  of  the  larger  animals,  it  would  still  be  im- 


© 


FIG.  37. — Diagram  illustrating  increase  of  volume  and  surface  with  increase  of 
diameter  of  sphere. 

possible  for  the  simple  external  surface  of  the  body  to  serve 
for  food  absorption,  because  of  the  well-known  relation 
between  the  surface  and  the  mass  of  a  solid  body.  When 
a  solid  body  in  the  form  of  a  sphere  increases  in  size,  its 
mass  or  volume  increases  as  the  cube  of  the  diameter,  while 
the  surface  increases  only  as  the  square  of  the  diameter 
(Fig.  37).  The  external  surface  of  minute  animals  a  few 
millimeters  in  diameter  can  take  up  enough  food  to  supply 
the  whole  body  mass.  But  among  large  animals  this  food- 
getting  surface  is  increased  as  the  square  of  the  diameter  of 


76  ANIMAL  LIFE 

the  body,  while  the  volume  or  food-using  surface  of  the 
body  is  increased  as  the  cube  of  its  diameter.  The  food  sup- 
plying can  not  keep  pace  with  the  food  using.  Hence  it  is 
absolutely  essential  that  among  large  animals  the  food-tak- 
ing surface  be  increased  so  that  it  will  remain  in  the  same 
favorable  proportion  to  the  mass  of  the  animal  as  is  the 
case  among  the  minute  animals,  where  the  simple  external 
body  surface  is  sufficient  to  obtain  all  the  food  necessary. 
This  increase  of  surface,  without  an  accompanying  increase 
of  size  of  the  animal,  is  accomplished  by  having  the  digest- 
ing and  assimilating  surface  inside  the  body  and  by  having 
it  greatly  folded.  The  surface  of  the  alimentary  canal  is, 
after  all,  simply  a  bent-in  continuation  of  the  outer  surface 
of  the  body.  It  is  open  to  the  outside  of  the  body  by  two 
openings,  and  wholly  closed  (except  by  its  porosity)  to  the 
true  inside  of  the  body.  By  the  bending  and  coiling  of 
the  alimentary  canal,  and  by  the  repeated  folding  of  its 
inner  wall,  the  alimentary  surface  is  greatly  increased. 
The  necessity  for  this  increase  accounts  largely  for  the 
complexity  of  the  alimentary  canal. 

But  it  is  not  alone  this  necessity  for  increased  surface 
that  accounts  for  the  great  specialization  of  the  alimentary 
canal  in  such  animals  as  the  insects  and  the  vertebrates. 
The  structural  differences  in  different  portions  of  the  canal, 
resulting  in  the  differentiation  of  the  canal  into  distinct 
parts,  or  the  differentiation  of  the  whole  organ  into  distinct 
subordinate  organs,  each  with  a  special  work  or  function  to 
perform,  are  the  result  of  the  necessity  for  the  special 
manipulation  of  the  special  kinds  of  foods  taken.  Animals 
which  feed  on  other  animals  must  have  mouth  structures 
fit  for  seizing  and  rending  their  prey,  and  the  alimentary 
canal  must  be  specially  modified  for  the  digestion  of  flesh. 
Animals  which  feed  on  vegetable  substances  must  have 
special  modifications  of  the  alimentary  canal  quite  different 
from  those  of  the  carnivores.  Some  insects,  like  the  mos- 
quito, take  only  liquid  food,  the  sap  of  plants,  or  the  blood 


FUNCTION  AND  STRUCTURE  YY 

of  animals ;  others,  like  the  weevils,  feed  on  the  hard,  dry 
substance  of  seeds  and  grains  ;  others,  like  the  grasshop- 
pers and  caterpillars,  eat  green  leaves  ;  and  still  others  eat 
other  insects.  The  alimentary  canal  of  each  of  these  kinds 
of  insects  differs  more  or  less  from  that  of  the  other  kinds. 
The  specialization  of  the  alimentary  canal  depends  then 
upon  the  necessity  for  a  large  food-digesting  and  absorbing 
surface,  and  on  the'  complex  treatment  of  the  food.  The 
character  of  this  specialization  in  each  case  depends  upon 
the  special  kind  or  quality  of  food  taken  by  the  animal  in 
question. 

45.  The  mutual  relation  of  function  and  structure. — The 
structure  of  an  animal  depends  upon  the  manner  in  which 
the  life  processes  or  functions  of  the  animal  are  performed. 
If  the  functions  are  performed  in  a  complex  manner,  the 
structure  of  the  body  is  complex  ;  if  the  functions  are  per- 
formed in  simple  manner,  the  body  will  be  simple  in  struc- 
ture. With  the  increase  in  degree  of  the  division  of  labor 
among  various  parts  of  the  body,  there  is  an  increase  in 
definiteness  and  extent  of  differentiation  of  structure. 
Each  part  or  organ  of  the  body  becomes  more  modified  and 
better  fitted  to  perform  its  own  special  function.  A  pecul- 
iar structural  condition  of  any  part  of  the  body,  or  of  the 
whole  body  of  any  animal,  is  not  to  be  looked  on  as  a  freak 
of  Xature,  or  as  a  wonder  or  marvel.  Such  a  structure  has 
a  significance  which  may  be  sought  for.  The  unusual 
structural  condition  is  associated  with  some  special  habit 
or  manner  of  performance  of  a  function.  Function  and 
structure  are  always  associated  in  Xature,  and  should  always 
be  associated  in  our  study  of  Nature. 


CHAPTER  V 

THE   LIFE   CYCLE 

46.  Birth,  growth  and  development,  and  death. — Certain 
phenomena  are  familiar  to  us  as  occurring  inevitably  in  the 
life  of  every  animal.     Each  individual  is  born  in  an  imma- 
ture or  young  condition  ;  it  grows  (that  is,  it  increases  in 
size),  and  develops  (that  is,  changes  more  or  less  in  struc- 
ture), and  dies.     These  phenomena  occur  in  the  succession 
of  birth,  growth  and  development,  and  death.     But  before 
any  animal  appears  to  us  as  an  independent  individual — 
that  is,  outside  the  body  of  the  mother  and  outside  of  an 
egg  (i.  e.,  before  birth  or  hatching,  as  we  are  accustomed  to 
call  such  appearance) — it  has  already  undergone  a  longer 
or  shorter  period  of  life.     It  has  been  a  new  living  organ- 
ism hours  or  days  or  months,  perhaps,  before  its  appear- 
ance to  us.     This  period  of  life  has  been  passed  inside  an 
egg,  or  as  an  egg  or  in  the  egg  stage,  as  it  is  variously 
termed.     The  life  of  an  animal  as  a  distinct  organism  be- 
gins in  an  egg.     And  the  true  life  cycle  of  an  organism  is 
its  life  from  egg  through  birth,  growth  and  development, 
and  maturity  to  the  time  it  produces  new  organisms  in 
the  condition  of  eggs.     The  life  cycle  is  from  egg  to  egg. 
Birth  and  growth,  two  of  the  phenomena  readily  apparent 
to  us  in  the  life  of  every  animal,  are  two  phenomena  in  the 
true  life  cycle.     Death  is  a  third  inevitable  phenomenon  in 
the  life  of  each  individual,  but  it  is  not  a  part  of  the  cycle. 
It  is  something  outside. 

47.  Life  cycle  of  simplest  animals. — The  simplest  animals 
have  no  true  egg  stage,  nor  perhaps  have  they  any  true 

78 


THE  LIFE  CYCLE 


79 


death.  The  new  Amoebae  are  from  their  beginning  like  the 
full-grown  Amoeba,  except  as  regards  size.  And  the  old 
Amoeba  does  not  die,  because  its  whole  body  continues  to 
live,  although  in  two  parts — the  two  new  Amoebae.  The  life 
cycle  of  the  simplest  animals  includes  birth  (usually  by 
simple  fission  of  the  body  of  the  parent),  growth,  and  some, 
but  usually  very  little,  development,  and  finally  the  repro- 
duction of  new  individuals,  not  by  the  formation  of  eggs, 
but  by  direct  division  of  the  body. 

48.  The  egg. — In  our  study  of  the  multiplication  of  ani- 
mals (Chapter  III)  we  learned  that  it  is  the  almost  univer- 


FIG.  38. — Eggs  of  different  animals  showing  variety  in  external  appearance,  a,  egg 
of  bird  ;  b,  eggs  of  toad ;  c,  egg  of  fish ;  d,  egg  of  butterfly  ;  e,  eggs  of  katydid 
on  leaf  ;  /,egg-case  of  skate. 

sal  rule  among  many-celled  animals  that  each  individual 
begins  life  as  a  single  cell,  which  has  been  produced  by  the 


80  ANIMAL   LIFE 

fusion  of  two  germ  cells,  a  sperm  cell  from  a  male  indi- 
vidual of  the  species  and  an  egg  cell  from  a  female  indi- 
vidual of  the  species.  The  single  cell  thus  formed  is  called 
the  fertilized  egg  cell,  and  its  subsequent  development 
results  in  the  formation  of  a  new  individual  of  the  same 
species  with  its  parents.  Now,  in  the  development  of  this 
cell  into  a  new  animal,  food  is  necessary,  and  sometimes  a 
certain  amount  of  warmth.  So  with  the  fertilized  egg  cell 
there  is,  in  the  case  of  all  animals  that  lay  eggs,  a  greater 
or  less  amount  of  food  matter — food  yolk,  it  is  called — gath- 
ered about  the  germ  cell,  and  both  germ  cell  and  food  yolk 
are  inclosed  in  a  soft  or  hard  wall.  Thus  is  composed  the 
egg  as  we  know  it.  The  hen's  egg  is  as  large  as  it  is  be- 
cause of  the  great  amount  of  food  yolk  it  contains.  The 
egg  of  a  fish  as  large  as  a  hen  is  much  smaller  than  the 
hen's  eg£ ,  it  contains  less  food  yolk.  Eggs  (Fig.  38)  may 
vary  also  in  their  external  appearance,  because  of  the  dif- 
ferent kinds  of  membrane  or  shells  which  may  inclose  and 
protect  them.  Thus  the  frog's  eggs  are  inclosed  in  a  thin 
membrane  and  imbedded  in  a  soft,  jelly-like  substance ; 
the  skate's  egg  has  a  tough,  dark-brown  leathery  inclosing 
wall ;  the  spiral  egg  of  the  bull-head  sharks  is  leathery  and 
colored  like  the  dark-olive  seaweeds  among  which  it  lies ; 
and  a  bird's  egg  has  a  hard  shell  of  carbonate  of  lime.  But 
in  each  case  there  is  the  essential  fertilized  germ  cell ;  in 
this  the  eggs  of  hen  and  fish  and  butterfly  and  cray-fish  and 
worm  are  alike,  however  much  they  may  differ  in  size  and 
external  appearance. 

49.  Embryonic  and  post-embryonic  development. — Some 
animals  do  not  lay  eggs,  that  is  they  do  not  deposit  the  fer- 
tilized egg  cell  outside  of  the  body,  but  allow  the  develop- 
ment of  the  new  individual  to  go  on  inside  the  body  of  the 
mother  for  a  longer  or  shorter  period.  The  mammals  and 
some  other  animals  have  this  habit.  When  such  an  ani- 
ma7  issues  from  the  body  of  the  mother,  it  is  said  to  be 
born.  When  the  developing  animal  issues  from  an  egg 


THE  LIFE  CYCLE 


81 


which  has  been  deposited  outside  the  body  of  the  mother, 
it  is  said  to  hatch.  The  animal  at  birth  or  at  time  of  hatch- 
ing is  not  yet  fully  developed.  Only  part  of  its  development 
or  period  of  immaturity  is  passed  within  the  egg  or  within 
the  body  of  the  mother.  That  part  of  its  life  thus  passed 
within  the  egg  or  mother's  body  is  called  the  embryonic  life 
or  embryonic  stages  of  development ;  while  that  period  of 
development  or  immaturity  from  the  time  of  birth  or  hatch- 
ing until  maturity  is  reached  is  called  the  post-embryonic 
life  or  post-embryonic  stages  of  development. 

50.  First  stages  in  development. — The  embryonic  develop- 
ment is  from  the  beginning  up  to  a  certain  point  practically 
identical  for  all  many-celled  animals — that  is,  there  are  cer- 


FIG.  39. — First  stages  in  embryonic  development  of  the  pond  snail  (Lymnceus).  a. 
egg  cell ;  b,  first  cleavage  ;  c,  second  cleavage  ;  d,  third  cleavage  ;  «,  after  numer- 
ous cleavages ;  /,  blastula  (in  section) ;  g,  gastrula,  just  forming  (in  section)  ; 
k,  gastrula,  completed  (in  section).— After  RABL. 

tain  principal  or  constant  characteristics  of  the  beginning 
development  which  are  present  in  the  development  of  all 
many-celled  animals.  The  first  stage  or  phenomenon  of 
development  is  the  simple  fission  of  the  germ  cell  into 
halves  (Fig.  39,  b).  These  two  daughter  cells  next  divide  so 
that  there  are  four  cells  (Fig.  39,  c)  ;  each  of  these  divides, 

and  this  division  is  repeated  until  a  greater  or  lesser  num- 

7 


82  ANIMAL  LIFE 

ber  (varying  with  the  various  species  or  groups  of  animals) 
of  cells  is  produced  (Fig.  39,  d).  The  phenomenon  of  re- 
peated division  of  the  germ  cell,  and  usually  the  surround- 
ing yolk,  is  called  cleavage,  and  this  cleavage  is  the  first 
stage  of  development  in  the  case  of  all  many-celled  animals. 
The  first  division  of  the  germ  cell  produces  usually  two  equal 
cells,  but  in  some  of  the  later  divisions  the  new  cells  formed 
may  not  be  equal.  In  some  animals  all  the  cleavage  cells  are 
of  equal  size ;  in  some  there  are  two  sizes  of  cells.  The  germ 
or  embryo  animal  consists  now  of  a  mass  of  few  or  many 
undifferentiated  primitive  cells  lying  together  and  usually 
forming  a  sphere  (Fig.  39,  e),  or  perhaps  separated  and  scat- 
tered through  the  food  yolk  of  the  egg.  The  next  stage  of  de- 
velopment is  this  :  the  cleavage  cells  arrange  themselves  so 
as  to  form  a  hollow  sphere  or  ball,  the  cells  lying  side  by  side 
to  form  the  outer  circumferential  wall  of  this  hollow  sphere 
(Fig.  39,/).  This  is  called  the  Uastula  or  blastoderm  stage 
of  development,  and  the  embryo  itself  is  called  the  blastula 
or  blastoderm.  This  stage  also  is  common  to  all  the  many- 
celled  animals.  The  next  stage  in  embryonic  development 
is  formed  by  the  bending  inward  of  a  part  of  the  blasto- 
derm cell  layer,  as  shown  in  Fig.  39,  g.  This  bending  in 
may  produce  a  small  depression  or  groove  ;  but  whatever  the 
shape  or  extent  of  the  sunken-in  part  of  the  blastoderm,  it 
results  in  distinguishing  the  blastoderm  layer  into  two 
parts,  a  sunken-in  portion  called  the  endoUast  and  the 
other  unmodified  portion  called  the  ectoblast.  Endo-  means 
"  within,"  and  the  cells  of  the  endoblast  often  push  so  far 
into  the  original  blastoderm  cavity  as  to  come  into  contact 
with  the  cells  of  the  ectoblast  and  thus  obliterate  this  cavity 
(Fig.  39,  Ti).  This  third  well-marked  stage  in  the  embry- 
onic development  is  called  the  gastrula  *  stage,  and  it  also 


*  This  gastrula  stage  is  not  always  formed  by  a  bending  in  or  in- 
vagination  of  the  blastoderm,  but  in  some  animals  is  formed  by  the 
splitting  off  or  delamination  of  cells  from  a  definite  limited  region  of 


THE  LIFE  CYCLE  83 

occurs  in  the  development  of  all  or  nearly  all  many-celled 
animals. 

51.  Continuity  of  development. — In  the  case  of  a  few  of 
the  simple  many-celled  animals  the  embryo  hatches — that 
is,  issues  from  the  egg  at  the  time  of  or  very  soon  after 
reaching  the  gastrula  stage.  In  the  higher  animals,  how- 
ever, development  goes  on  within  the  egg  or  within  the 
body  of  the  mother  until  the  embryo  becomes  a  complex 
body,  composed  of  many  various  tissues  and  organs.  Al- 
most all  the  development  may  take  place  within  the  egg, 


a 

FIG.  40.— Honey-bee,    a,  adult  worker  ;  b,  young  or  larval  worker. 

so  that  when  the  young  animal  hatches  there  is  necessary 
little  more  than  a  rapid  growth  and  increase  of  size  to 
make  it  a  fully  developed,  mature  animal.  This  is  the  case 
with  the  birds :  a  chicken  just  hatched  has  most  of  the 
tissues  and  organs  of  a  full-grown  fowl,  and  is  simply  a 
little  hen.  But  in  the  case  of  other  animals  the  young 
hatches  from  the  egg  before  it  has  reached  such  an  ad- 
vanced stage  of  development ;  a  young  star-fish  or  young 
crab  or  young  honey-bee  (Fig.  40)  just  hatched  looks  very 
different  from  its  parent.  It  has  yet  a  great  deal  of  devel- 
opment to  undergo  before  it  reaches  the  structural  condi- 
tion of  a  fully  developed  and  fully  grown  star-fish  or  crab 
or  bee.  Thus  the  development  of  some  animals  is  almost 

the  blastoderm.  Our  knowledge  of  gastrulation  and  the  gastrula  stage 
is  yet  far  from  complete.  % 


84  ANIMAL  LIFE 

wholly  embryonic  development — that  is,  development  with- 
in the  egg  or  in  the  body  of  the  mother — while  the  devel- 
opment of  other  animals  is  largely  post-embryonic  or  larval 
development,  as  it  is  often  called.  There  is  no  important 
difference  between  embryonic  and  post-embryonic  develop- 
ment. The  development  is  continuous  from  egg  cell  to 
mature  animal,  and  whether  inside  or  outside  of  an  egg  it 
goes  on  regularly  and  uninterruptedly. 

52.  Development  after  the  gastrula  stage. — The  cells  which 
compose  the  embryo  in  the  cleavage  stage  and  blastoderm 
stage,  and  even  in  the  gastrula  stage,  are  all  similar ;  there 
is  little  or  no  differentiation  shown  among  them.     But  from 
the  gastrula  stage  on  development  includes  three  important 
things :   the  gradual  differentiation  of  cells  into  various 
kinds  to  form  the  various  kinds  of  animal   tissues ;  the 
arrangement  and  grouping  of  these  cells  into  organs  and 
body  parts ;  and  finally  the  developing  of  these    organs 
and  body  parts  into  the  special  condition  characteristic  of 
the  species  of  animal  to  which  the  developing  individual 
belongs.     From  the  primitive  undifferentiated  cells  of  the 
blastoderm,  development  leads  to  the  special  cell  types  of 
muscle  tissue,  of  bone  tissue,  of  nerve  tissue ;  and  from  the 
generalized  condition  of  the  embryo  in  its  early  stages  de- 
velopment leads  to  the  specialized  condition  of  the  body  of 
the  adult  animal.     Development  is  from  the  general  to  the 
special,  as  was  said  years  ago  by  the  first  great  student  of 
development. 

53.  Divergence  of  development, — A  star-fish,  a  beetle,  a 
dove,  and  a  horse  are  all  alike  in  their  beginning — that  is, 
the  body  of  each  is  composed  of  a  single  cell,  a  single  struc- 
tural unit.     And  they  are  all  alike,  or  very  much  alike, 
through  several  stages  of  development ;  the  body  of  each 
is  first  a  single  cell,  then  a  number  of  similar  undifferen- 
tiated cells,  and  then  a  hollow  sphere  consisting  of  a  single 
layer  of  similar  undifferentiated  cells.     But  soon  in  the 
course  of  development  the  embryos  begin  to  differ,  and  as 


OF  THE 

UNIVERSITY   ) 

OF  / 

THE  LIFE  CYCLE  85 

the  young  animals  get  further  and  further  along  in  the 
course  of  their  development,  they  become  more  and  more 
diiferent  until  each  finally  reaches  its  fully  developed  ma- 
ture form,  showing  all  the  great  structural  differences  be- 
tween the  star-fish  and  the  dove,  the  beetle  and  the  horse. 
That  is,  all  animals  begin  development  alike,  but  gradually 
diverge  from  each  other  during  the  course  of  development. 
There  are  some  extremely  interesting  and  significant 
things  about  this  divergence  to  which  attention  should  be 
given.  While  all  animals  are  alike  structurally*  at  the 
beginning  of  development,  so  far  as  we  can  see,  they  do  not 
all  differ  at  the  time  of  the  first  divergence  in  development. 
This  first  divergence  is  only  to  be  noted  between  two  kinds 
of  animals  which  belong  to  different  great  groups  or  classes. 
But  two  animals  of  different  kinds,  both  belonging  to  some 
one  great  group,  do  not  show  differences  until  later  in  their 
development.  This  can  best  be  understood  by  an  example. 
All  the  butterflies  and  beetles  and  grasshoppers  and  flies 
belong  to  the  great  group  of  animals  called  Insecta,  or  in- 
sects. There  are  many  different  kinds  of  insects,  an4  these 
kinds  can  be  arranged  in  subordinate  groups,  such  as  the 
Diptera,  or  flies,  the  Lepidoptera,  or  butterflies  and  moths, 
and  so  on.  But  all  have  certain  structural  characteristics 
in  common,  so  that  they  are  comprised  in  one  great  group 
or  class — the  Insecta.  Another  great  group  of  animals  is 
known  as  the  Vertebrata,  or  back-boned  animals.  The  class 
Vertebrata  includes  the  fishes,  the  batrachians,  the  reptiles, 
the  birds,  and  the  mammals,  each  composing  a  subordinate 
group,  but  all  characterized  by  the  possession  of  a  back- 

*  They  are  alike  structurally,  when  we  consider  the  cell  as  the  unit 
of  animal  structure.  That  the  egg  cells  of  diiferent  animals  may  dif- 
fer in  their  fine  or  ultimate  structure,  seems  certain.  For  each  one  of 
these  egg  cells  is  destined  to  become  some  one  kind  of  animal,  and  no 
other ;  each  is,  indeed,  an  individual  in  simplest,  least  developed  con- 
dition of  some  one  kind  of  animal,  and  we  must  believe  that  difference 
in  kind  of  animals  depends  upon  difference  in  structure  in  the  egg  itself. 


86  ANIMAL  LIFE 

bone,  or,  more  accurately  speaking,  of  a  notochord,  a  back- 
bone-like structure.  Now,  an  insect  and  a  vertebrate  di- 
verge very  soon  in  their  development  from  each  other ;  but 
two  insects,  such  as  a  beetle  and  a  honey-bee,  or  any  two 
vertebrates,  such  as  a  frog  and  a  pigeon,  do  not  diverge 
from  each  other  so  soon.  That  is,  all  vertebrate  animals 
diverge  in  one  direction  from  the  other  great  groups,  but 
all  the  members  of  the  great  group  keep  together  for  some 
time  longer.  Then  the  subordinate  groups  of  the  Verte- 
brata,  such  as  the  fishes,  the  birds,  and  the  others  diverge, 
and  still  later  the  different  kinds  of  animals  in  each  of 
these  groups  diverge  from  each  other.  In  the  illustration 
(Fig.  41)  on  the  opposite  page  will  be  seen  pictures  of  the 
embryos  of  various  vertebrate  animals  shown  as  they  appear 
at  different  stages  or  times  in  the  course  of  development. 
The  embryos  of  a  fish,  a  salamander,  a  tortoise,  a  bird,  and 
a  mammal,  representing  the  five  principal  groups  of  the 
Vertebrata,  are  shown.  In  the  upper  row  the  embryos  are 
in  the  earliest  of  all  the  stages  figured,  and  they  are  very 
much  alike.  They  show  no  obvious  characteristics  of 
fish  or  bird.  Yet  there  are  distinctive  characteristics  of 
the  great  class  Vertebrata.  Any  of  these  embryos  could 
readily  be  distinguished  from  an  embryonic  insect  or  worm 
or  sea-urchin.  In  the  second  row  there  is  beginning  to  be 
manifest  a  divergence  among  the  different  embryos,  al- 
though it  would  still  be  a  difficult  matter  to  distinguish 
certainly  which  was  the  young  fish  and  which  the  young 
salamander,  or  which  the  young  tortoise  and  which  the 
young  bird.  In  the  bottom  row,  showing  the  animals  in  a 
later  stage  of  development,  the  divergence  has  proceeded 
so  far  that  it  is  now  plain  which  is  a  fish,  which  batrachian, 
which  reptile,  which  bird,  and  which  mammal. 

54.  The  laws  or  general  facts  of  development.— That  the 
course  of  development  of  any  animal  from  its  beginning  to 
fully  developed  adult  form  is  fixed  and  certain  is  readily 
seen.  Every  rabbit  develops  in  the  same  way  ;  every  grass- 


ilcirn cinder 

Jortofse    Chick 

.  4i._ Different  vertebrate  animal  in  successive  embryonic  stages.  I,  first 
or  earliest  of  the  stages  figured  ;  II,  second  of  the  stages ;  HI,  third  or 
latest  of  the  stages.— After  HAECKBL. 


38  ANIMAL  LIFE 

hopper  goes  through  the  same  developmental  changes  from 
single  egg  cell  to  the  full-grown  active  hopper  as  every 
other  grasshopper  of  the  same  kind — that  is,  development 
takes  place  according  to  certain  natural  laws,  the  laws  of 
animal  development.  These  laws  may  be  roughly  stated  as 
follows  :  All  many-celled  animals  begin  life  as  a  single  cell, 
the  fertilized  egg  cell ;  each  animal  goes  through  a  certain 
orderly  series  of  developmental  changes  which,  accom- 
panied by  growth,  leads  the  animal  to  change  from  single 
cell  to  the  many-celled,  complex  form  characteristic  of  the 
species  to  which  the  animal  belongs ;  this  development  is 
from  simple  to  complex  structural  condition ;  the  develop- 
ment is  the  same  for  all  individuals  of  one  species.  While 
all  animals  begin  development  similarly,  the  course  of  devel- 
opment in  the  different  groups  soon  diverges,  the  diver- 
gence being  of  the  nature  of  a  branching,  like  that  shown 
in  the  growth  of  a  tree.  In  the  free  tips  of  the  smallest 
branches  we  have  represented  the  various  species  of  ani- 
mals in  their  fully  developed  condition,  all  standing  clearly 
apart  from  each  other.  But  in  tracing  back  the  develop- 
ment of  any  kind  of  animal,  we  soon  come  to  a  point  where 
it  very  much  resembles  or  becomes  apparently  identical 
with  some  other  kind  of  animal,  and  going  further  back  we 
find  it  resembling  other  animals  in  their  young  condition, 
and  so  on  until  we  come  to  that  first  stage  of  development, 
that  trunk  stage,  where  all  animals  are  structurally  alike. 
To  be  sure,  any  animal  at  any  stage  in  its  existence  differs 
absolutely  from  any  other  kind  of  animal,  in  that  it  can 
develop  into  only  its  own  kind  of  animal.  There  is  some- 
thing inherent  in  each  developing  animal  that  gives  it  an 
identity  of  its  own.  Although  in  its  young  stages  it  may  be 
hardly  distinguishable  from  some  other  kind  of  animal  in 
similar  stages,  it  is  sure  to  come  out,  when  fully  developed, 
an  individual  of  the  same  kind  as  its  parents  were  or  are. 
The  young  fish  and  the  young  salamander  in  the  upper  row 
in  Fig.  41  seem  very  much  alike,  but  one  embryo  is  sure  to 


THE  LIFE  CYCLE  89 

develop  into  a  fish  and  the  other  into  a  salamander.  This 
certainty  of  an  embryo  to  become  an  individual  of  a  certain 
kind  is  called  the  law  of  heredity.  • 

55.  The  significance  of  the  facts  of  development.  —  The 
significance  of  the  developmental  phenomena  is  a  matter 
about  which  naturalists  have  yet  very  much  to  learn.  It  is 
believed,  however,  by  practically  all  naturalists  that  many 
of  the  various  stages  in  the  development  of  an  animal  cor- 
respond to  or  repeat  the  structural  condition  of  the  ani- 
mal's ancestors.  Naturalists  believe  that  all  backboned  or 
vertebrate  animals  are  related  to  each  other  through  being 
descended  from  a  common  ancestor,  the  first  or  oldest 
backboned  animal.  In  fact,  it  is  because  all  these  back- 
boned animals — the  fishes,  the  batrachians,  the  reptiles,  the 
birds,  and  the  mammals — have  descended  from  a  common 
ancestor  that  they  all  have  a  backbone.  It  is  believed  that 
the  descendants  of  the  first  backboned  animal  have  in  the 
course  of  many  generations  branched  off  little  by  little  from 
the  original  type  until  there  have  come  to  exist  very  real  and 
obvious  differences  among  the  backboned  animals — differ- 
ences which  among  the  living  backboned  animals  are  familiar 
to  all  of  us.  The  course  of  development  of  an  individual  ani- 
mal is  believed  by  many  naturalists  to  be  a  very  rapid,  and 
evidently  much  condensed  and  changed,  recapitulation  of 
the  history  which  the  species  or  kind  of  animal  to  which  the 
developing  individual  belongs  has  passed  through  in  the 
course  of  its  descent  through  a  long  series  of  gradually  chang- 
ing ancestors.  If  this  is  true,  then  we  can  readily  under- 
stand why  the  fish  and  the  salamander  and  tortoise  and 
bird  and  rabbit  are  all  so  much  alike  in  their  earlier  stages 
of  development,  and  gradually  come  to  differ  more  and 
more  as  they  pass  through  later  and  later  developmental 
stages. 

Some  naturalists  believe  that  the  ontogenetic  stages  are 
not  as  significant  in  throwing  light  upon  the  evolutionary 
history  of  the  species  as  just  indicated.  Some  think  that 


90  ANIMAL  LIFE 

when  the  earlier  stages  of  one  species  correspond  pretty 
closely  with  the  early  stages  of  another,  we  have  a  good 
basis  for  making  up  our  minds  about  relationship  between 
the  two  species.  But  it  is  certainly  not  obvious  why  we 
should  have  a  similarity  among  the  younger  stages  of  dif- 
ferent animals  and  no  correspondence  among  the  older 
stages  of  more  recent  animals  with  the  younger  stages  of 
more  ancient  ones.  But  on  the  other  hand  it  is  certainly 
true  that  a  too  specific  application  of  the  broad  generaliza- 
tion that  ontogeny  repeats  phylogeny  has  led  to  numerous 
errors  of  interpreting  genealogic  relationship. 

56.  Metamorphosis. — While  a  young  robin  when  it  hatches 
from  the  egg  or  a  young  kitten  at  birth  resembles  its  par- 
ents, a  young  star-fish  or  a  young  crab  or  a  young  butterfly 
when  hatched  does  not  at  all  resemble  its  parents.     And 
while  the  young  robin  after  hatching  becomes  a  fully  grown 
robin  simply  by  growing  larger  and  undergoing  compara- 
tively slight  developmental  changes,  the  young  star-fish  or 
young  butterfly  not  only  grows  larger,  but  undergoes  some 
very  striking  developmental  changes;   the  body  changes 
very  much  in  appearance.     Marked  changes  in  the  body  of 
an  animal  during  post-embryonic  or  larval   development 
constitute  what  is  called  metamorpliic  development,  or  the 
animal  is  said  to  undergo  or  to  show  metamorphosis  in  its 
development.     Metamorphosis  is  one  of  the  most  interest- 
ing features  in  the  life  history  or  development  of  animals, 
and  it  can  be,  at  least  as  far  as  its  external  aspects  are  con- 
cerned, very  readily  observed  and  studied. 

57.  Metamorphosis  among  insects. — All  the  butterflies  and 
moths  show  metamorphosis  in  their  development.     So  do 
many  other  insects,  as  the  ants,  bees,  and  wasps,  and  all  the 
flies  and  beetles.     On  the  other  hand,  many  insects  do  not 
show  metamorphosis,  but,  like  the  birds,  are  hatched  from 
the  egg  in  a  condition  plainly  resembling  the  parents.     A 
grasshopper  (Fig.  42)  is  a  convenient  example  of  an  insect 
without  metamorphosis,  or  rather,  as  there  are,  after  all, 


THE  LIFE  CYCLE 


91 


a  few  easily  perceived  changes  in  its  post-embryonic  devel- 
opment, of  an  insect  with  an  "incomplete  metamorpho- 
sis." The  eggs  of  grasshoppers  are  laid  in  little  packets 
of  several  score  half  an  inch  below  the  surface  of  the 
ground.  When  the  young  grasshopper  hatches  from  the 
egg  it  is  of  course  very  small,  but  it  is  plainly  recognizable 
as  a  grasshopper.  But  in  one  important  character  it  dif- 
fers from  the  adult,  and  that  is  in  its  lack  of  wings.  The 
adult  grasshopper  has  two  pairs  of  wings ;  the  just  hatched 
young  or  larval  grasshopper  has  no  wings  at  all.  The 
young  grasshopper  feeds  voraciously  and  grows  rapidly. 


FIG.  42. —Post-embryonic  development  (incomplete  metamorphosis)  of  the  Rockj" 
Mountain  locust  (Melanoplus  spretus).  a,  b,  c,  d,  e,  and  /,  successive  develop- 
mental stages  from  just  hatched  to  adult  individual.— After  EMERTON. 


In  a  few  days  it  molts,  or  casts  its  outer  skin  (not  the 
true  skin,  but  a  thin,  firm  covering  or  outer  body  wall  com- 
posed of  a  substance  called  chitin,  which  is  secreted  by  the 
cells  of  the  true  skin).  In  this  second  larval  stage  there 
can  be  seen  the  rudiments  of  four  wings,  in  the  condition 
of  tiny  wing  pads  on  the  back  of  the  middle  part  of  the 
body  (the  thorax).  Soon  the  chitinous  body  covering  is 
shed  again,  and  after  this  molt  the  wing  pads  are  mark- 
edly larger  than  before.  Still  another  molt  occurs,  with 
another  increase  in  size  of  the  developing  wings,  and  after 
a  fifth  and  last  molt  the  wings  are  fully  developed,  and 


92 


ANIMAL  LIFE 


the  grasshopper  is  no  longer  in  a  larval  or  immature  condi- 
tion, but  is  full  grown  and  adult. 

For  example  of  complete  metamorphosis  among  insects 
we  may  choose  a  butterfly,  the  large  red-brown  butterfly 


FIG.  43.— Metamorphosis  of  monarch  butterfly  (Anosia  plexippus). 
c,  pupa ;  d,  imago  or  adult. 


o,  egg  ;  b,  larva  ; 


common  in  the  United  States  and  called  the  monarch  or 
milkweed  butterfly  (Anosia  plexippus).  The  eggs  (Fig. 
43,  a)  of  this  butterfly  are  laid  on  the  leaves  of  various  kinds 
of  milkweed  (Asclepias).  The  larval  butterfly  or  butterfly 
larva  or  caterpillar  (as  the  first  young  stage  of  the  butter- 


THE  LIFE  CYCLE 


93 


flies  and  moths  is  usually  called),  which  hatches  from  the 
egg  in  three  or  four  days,  is  a  creature  hearing  little  or 
no  resemblance  to  the  beautiful  winged  adult.  The  larva 
is  worm-like,  and  instead  of  having  three  pairs  of  legs 
like  the  butterfly  it  has  eight  pairs;  it  has  biting  jaws 
in  its  mouth  with  which  it  nips  off  bits  of  the  green  milk- 
weed leaves,  instead  of  having  a  long,  slender,  sucking 
proboscis  for  drinking  flower  nectar  as  the  butterfly  has. 
The  body  of  the  crawl- 
ing worm-like  larva 
(Fig.  43,  V)  is  greenish 
yellow  in  color,  with 
broad  rings  or  bands  of 
shining  black.  It  has 
no  wings,  of  course.  It 
eats  voraciously,  grows 
rapidly  and  molts.  But 
after  the  molting  there 
is  no  appearance  of 
rudimentary  wings;  it 
is  simply  a  larger  worm- 
like  larva.  It  continues 
to  feed  and  grow,  molt- 
ing several  times,  until 
after  the  fourth  molt  it 
appears  no  longer  as  an 
active,  crawling,  feed- 
ing, worm-like  larva,  but  as  a  quiescent,  non-feeding  pupa 
or  chrysalis  (Fig.  43,  c).  The  immature  butterfly  is  now 
greatly  contracted,  and  the  outer  chitinous  wall  is  very 
thick  and  firm.  It  is  bright  green  in  color  with  golden  dots. 
It  is  fastened  by  one  end  to  a  leaf  of  the  milkweed,  where 
it  hangs  immovable  for  from  a  few  days  to  two  weeks. 
Finally,  the  chitin  wall  of  the  chrysalis  splits,  and  there 
issues  the  full-fledged,  great,  four-winged,  red-brown  butter- 
fly (Fig.  43,  d).  Truly  this  is  a  metamorphosis,  and  a  start- 


FIQ.  44.— Metamorphosis  of  mosquito  (Culex). 
a,  larva ;  b,  pupa. 


ANIMAL  LIFE 


ling  one.     But  we  know  that  development  in  other  animals 
is  a  gradual  and  continuous  process,  and  so  it  is  in  the 

case  of  the  butterfly. 
The  gradual  chang- 
ing is  masked  by  the 
outer  covering  of  the 
body  in  both  larva 
and  pupa.  It  is  only 
at  each  molting  or 
throwing  off  of  this 
unchanging,  unyield- 
ing chitin  armor  that 
we  perceive  how  far 
this  change  has  gone. 
The  longest  time  of 
concealment  is  that 
during  the  pupal  or 
chrysalis  stage,  and 
the  results  of  the 
changing  or  develop- 
ment when  finally  re- 
vealed by  the  split- 
ting of  the  pupal 
case  are  hence  the 
most  striking. 

58.  Metamorphosis  of  the  toad. — Metamorphosis  is  found 
in  the  development  of  numerous  other  animals,  as  well  as 
among  the  insects.  Certain  cases  are  familiar  to  all — the 
metamorphosis  of  the  frogs  and  toads  (Fig.  46).  The  eggs 
of  the  toad  are  arranged  in  long  strings  or  ribbons  in  a 
transparent  jelly-like  substance.  These  jelly  ribbons  with 
the  small,  black,  bead-like  eggs  in  them  are  wound  around 
the  stems  of  submerged  plants  or  sticks  near  the  shores  of 
the  pond.  From  each  egg  hatches  a  tiny,  wriggling  tad- 
pole, differing  nearly  as  much  from  a  full-grown  toad  as 
a  caterpillar  differs  from  a  butterfly.  The  tadpoles  feed  on 


FIG.  45.— Larva  of  a  butterfly  just  changing  into 
pupa  (making  last  larval  molt).  Photograph 
from  Nature. 


THE  LIFE  CYCLE 


95 


the  microscopic  plants  to  be  found  in  the  water,  and  swim 
easily  about  by  means  of  the  long  tail.  The  very  young 
tadpoles  remain  underneath  the  surface  of  the  water  all  the 
time,  breathing  the  air  which  is  mixed  with  water  by  means 
of  gills.  But  as  they  become  older  and  larger  they  come 
often  to  the  surface  of  the  water.  Lungs  are  developing 
inside  the  body,  and  the  tadpole  is  beginning  to  breathe  as 
a  land  animal,  although  it  still  breathes  partly  by  means  of 
gills,  that  is,  as  an  aquatic  animal.  Soon  it  is  apparent  that 
although  the  tadpole  is  steadily  and  rapidly  growing  larger, 
its  tail  is  growing  shorter  and  smaller  instead  of  larger.  At 
the  same  time,  fore  and  hind  legs  bud  out  and  rapidly  take 


FIG.  46.— Metamorphosis  of  the  toad  (partly  after  GAGE).    At  left  the  strings  of  eggs, 
in  water  the  various  tadpole  or  larval  stages,  and  on  bank  the  adult  toads. 

form  and  become  functional.  By  the  time  that  the  tail 
gets  very  short,  indeed,  the  young  toad  is  ready  to  leave  the 
water  and  live  as  a  land  animal.  On  land  the  toad  lives,  as 


96 


ANIMAL  LIFE 


we  know,  on  insects  and  snails  and  worms.  The  metamor- 
phosis of  the  toad  is  not  so  striking  as  that  of  the  butter- 
fly, but  if  the  tadpole  were  inclosed  in  an  unchanging 
opaque  body  wall  while  it  was  losing  its  tail  and  getting  its 
legs,  and  this  wall  were  to  be  shed  after  these  changes  were 
made,  would  not  the  metamorphosis  be  nearly  as  extraordi= 


Fia.  47.— Metamorphosis  of  sea- 
urchin.  Upper  figure  the  adult, 
lower  figure  the  pluteus  larva. 


nary  as  in  the  case  of 
the  butterfly?  But  in 
the  metamorphosis  of 
the  toad  we  can  see  the 
gradual  and  continuous 
character  of  the  change. 

59.  Metamorphosis  among  other  animals. — Many  other 
animals,  besides  insects  and  frogs  and  toads,  undergo  meta- 
morphosis. The  just-hatched  sea-urchin  does  not  resemble 
a  fully  developed  sea-urchin  at  all.  It  is  a  minute  worm- 
like  creature,  provided  with  cilia  or  vibratile  hairs,  by  means 
of  which  it  swims  freely  about.  It  changes  next  into  a  curi- 
ous bootjack-shaped  body  called  the  pluteus  stage  (Fig,  47). 
In  the  pluteus  a  skeleton  of  lime  is  formed,  and  the  final 
true  sea-urchin  body  begins  to  appear  inside  the  pluteus, 


THE  LIFE  CYCLE 


97 


developing  and  growing  by  using  up  the  body  substance  of 
the  pluteus.  Star-fishes,  which  are  closely  related  to  sea- 
urchins,  show  a  simi- 
lar metamorphosis, 
except  that  there  is 
no  pluteus  stage,  the 
true  star-fish-shaped 
body  forming,  with- 
in and  at  the  expense 
of  the  first  larval 
stage,  the  ciliated 
free-swimming  stage. 

A  young  crab  just 
issued  from  the  egg 
(Fig.  48)  is  a  very 
different  appearing 
creature  from  the 
adult  or  fully  devel- 
oped crab.  The  body 
of  the  crab  in  its 
first  larval  stage  is 
composed  of  a  short, 
globular  portion,  fur- 
nished with  conspicuous  long  spines  and  a  relatively  long, 
jointed  tail.  This  is  called  the  zoe'a  stage.  The  zoe'a 
changes  into  a  stage  called  the  megalops,  which  has  many 
characteristics  of  the  adult  crab  condition,  but  differs  espe- 
cially from  it  in  the  possession  of  a  long,  segmented  tail, 
and  in  having  the  front  half  of  the  body  longer  than  wide. 
The  crab  in  the  megalops  stage  looks  very  much  like  a 
tiny  lobster  or  shrimp,  The  tail  soon  disappears  and  the 
body  widens,  and  the  final  stage  is  reached. 

In  many  families  of  fishes  the  changes  which  take  place 

in  the  course  of  the  life  cycle  are  almost  as  great  as  in  the 

case  of  the  insect  or  the  toad.     In  the  lady-fish  (Albula 

vulpes)  the  very  young  (Fig.  49)  are  ribbon-like  in  form, 

8 


FIG. 


48. — Metamorphosis  of  the  crab,     a,  the  zoe'a 
stage ;  &,  the  megalops ;  c,  the  adult. 


98 


ANIMAL  LIFE 


with  small  heads  and  very  loose  texture  of  the  tissues,  the 
body  substance  being  jelly-like  and  transparent.  As  the  fish 
grows  older  the  body  becomes  more  compact,  and  therefore 


is 


Fio.  49.— Stages  in  the  post-embryonic  development  of  the  lady-fish  (Albula  vulpes), 
showing  metamorphosis.— After  C.  H.  GILBERT. 

shorter  and  slimmer.     After  shrinking  to  the  texture  of  an 
ordinary  fish,  its  growth  in  size  begins  normally,  although 


THE  LIFE  CYCLE 


99 


it  has  steadily  increased  in  actual  weight.  Many  herring, 
eels,  and  other  soft-bodied  fishes  pass  through  stages  simi- 
lar to  those  seen  in  the  lady-fish.  Another  type  of  devel- 
opment is  illustrated  in  the  sword-fish.  The  young  has  a 
bony  head,  bristling  with  spines.  As  it  grows  older  the 
spines  disappear,  the  skin  grows  smoother,  and,  finally,  the 
bones  of  the  upper  jaw  grow  together,  forming  a  prolonged 
sword,  the  teeth  are  lost  and  the  fins  become  greatly  modi- 
fied. Fig.  50  shows  three  of  these  stages  of  growth.  The 


a 


FIG.  50.— Three  stages  in  the  development  of  the  sword-fish  (Xiphias  gladius). 
a,  very  young ;  b,  older  ;  c,  adult.— Partly  after  LUTKBN. 

flounder  or  flat-fish  (Fig.  51)  when  full  grown  lies  flat  on 
one  side  when  swimming  or  when  resting  in  the  sand  on 
the  bottom  of  the  sea.  The  eyes  are  both  on  the  upper 
side  of  the  body,  and  the  lower  side  is  blind  and  colorless. 
When  the  flounder  is  hatched  it  is  a  transparent  fish,  broad 
and  flat,  swimming  vertically  in  the  water,  with  an  eye  on 
each  side.  As  its  development  (Fig.  52)  goes  on  it  rests 
itself  obliquely  on  the  bottom,  the  eye  of  the  lower  side 
turns  upward,  and  as  growth  proceeds  it  passes  gradually 


100 


ANIMAL   LIFE 


around  the  forehead,  its  socket  moving  with  it,  until  both 
eyes  and  sockets  are  transferred  by  twisting  of  the  skull  to 


FIG.  51.— The  wide-eyed  flounder  (Platophrys  lunatus).    Adult,  showing  both  eyes  on 
upper  side  of  head. 

the  upper  side.  In  some  related  forms  or  soles  the  small 
eye  passes  through  the  head  and  not  around  it,  appearing 
finally  in  the  same  socket  with  the  other  eye. 

Thus  in  almost  all  the  great  groups  of  animals  we  find 
certain  kinds  which  show  metamorphosis  in  their  post- 
embryonic  development.  But  metamorphosis  is  simply 
development;  its  striking  and  extraordinary  features  are 
usually  due  to  the  fact  that  the  orderly,  gradual  course  of 
the  development  is  revealed  to  us  only  occasionally,  with 
the  result  of  giving  the  impression  that  the  development  is 
proceeding  by  leaps  and  bounds  from  one  strange  stage  to 


FIG.  52. — Development  of  a  flounder  (after  EMERY).    The  eyes  in  the  young  flounder 
are  arranged  normally,  one  on  each  side  of  head. 

another.     If  metamorphosis  is  carefully  studied  it  loses  its 
aspect  of  marvel,  although  never  its  great  interest. 


THE  LIFE  CYCLE 


101 


60.  Duration  of  life.  —  After  an  animal  has  completed  its 
development  it  has  but  one  thing  to  do  to  complete  its  life 
cycle,  and  that  is  the  production  of  offspring.  When  it 
has  laid  eggs  or  given  birth  to  young,  it  has  insured  the 
beginning  of  a  new  life  cycle.  Does  it  now  die  ?  Is  the 
business  of  its  life  accomplished  ?  There  are  many  animals 
which  die  immediately  or  very  soon  after  laying  eggs.  The 
May-flies  —  ephemeral  insects  which  issue  as  winged  adults 
from  ponds  or  lakes  in  which 
they  have  spent  from  one  to 
three  years  as  aquatic  crawl- 
ing or  swimming  larvae,  nutter 
about  for  an  evening,  mate, 
drop  their  packets  of  fertil- 
ized eggs  into  the  water,  and 
die  before  the  sunrise  —  are 
extreme  examples  of  the  nu- 
merous kinds  of  animals 
whose  adult  life  lasts  only  long 
enough  for  mating  and  egg- 
laying.  But  elephants  live  for 
two  hundred  years.  Whales 
probably  live  longer.  A  horse 
lives  about  thirty  years,  and  so 
may  a  cat  or  toad.  A  sea- 
anemone,  which  was  kept  in  an  aquarium,  lived  sixty-six 
years.  Cray-fishes  may  live  twenty  years.  A  queen  bee 
was  kept  in  captivity  for  fifteen  years.  Most  birds  have 
long  lives  —  the  small  song  birds  from  eight  to  eighteen 
years,  and  the  great  eagles  and  vultures  up  to  a  hundred 
years  or  more.  On  the  other  hand,  among  all  the  thou- 
sands of  species  of  insects,  the  individuals  of  very  few  in- 
deed live  more  than  a  year  ;  the  adult  life  of  most  insects 
being  but  a  few  days  or  weeks,  or  at  best  months.  Even 
among  the  higher  animals,  some  are  very  short-lived. 
In  Japan  is  a  small  fish  (Solaux)  which  probably  lives 


"* 


102 


ANIMAL   LIFE 


but  a  year,  ascending  the  rivers  in  numbers  when  young  in 
the  spring,  the  whole  mass  of  individuals  dying  in  the  fall 
after  spawning. 

Naturalists  have  sought  to  discover  the  reason  for  these 
extraordinary  differences  in  the  duration  of  life*c*  different 
animals,  and  while  it  can  not  be  said  that  the  reason  or 
reasons  are  wholly  known,  yet  the  probability  is  strong  that 
the  duration  of  life  is  closely  connected  with,  or  dependent 
upon,  the  conditions  attending  the  production  of  offspring. 
It  is  not  sufficient,  as  we  have  learned  from  our  study  of 
the  multiplication  of  animals  (Chapter  III),  that  an  adult 
animal  shall  produce  simply  a  single  new  individual  of  its 
kind,  or  even  only  a  few.  It  must  produce  many,  or  if  it 
produces  comparatively  few  it  must  devote  great  care  to 
the  rearing  of  these  few,  if  the  perpetuation  of  the  species 
is  to  be  insured.  Now,  almost  all  long-lived  animals  are 
species  which  produce  but  few  offspring  at  a  tinie,  and 
reproduce  only  at  long  intervals,  while  most  short-lived  ani- 
mals produce  a  great  many  eggs,  and  these  all  at  one  time. 
Birds  are  long-lived  animals;  as  we  know,  most  of  them 
lay  eggs  but  once  a  year,  and  lay  only  a  few  eggs  each-time. 
Many  of  the  sea  birds  which  swarm  in  countless  numbers 
on  the  rocky  ocean  islets  and  great  sea  cliffs  lay  only  a 
single  egg  once  each  year.  And  these  birds,  the  guillemots 
and  murres  and  auks,  are  especially  long-lived.  Insects,  on 
the  contrary,  usually  produce  many  eggs,  and  all  of  them 
in  a  short  time.  The  May-fly,  with  its  one  evening's  lifetime, 
lets  fall  from  its  body  two  packets  of  eggs  and  then  dies. 
Thus  the  shortening  of  the  period  of  reproduction  with  the 
production  of  a  great  many  offspring  seem  to  be  always 
associated  with  a  short  adult  lifetime ;  while  a  long  period 
of  reproduction  with  the  production  of  few  offspring  at  a 
time  and  care  of  the  offspring  are  associated  with  a  long 
adult  lifetime. 

There  seems  also  to  be  some  relation  between  the  size 
of  animals  and  the  length  of  life.  As  a  general  rule, 


THE  LIFE  CYCLE  103 

large  animals  are  long-lived  and  small  animals  have  short 
lives. 

61.  Death. — At  the  end  comes  death.  After  the  animal 
has  completed  its  life  cycle,  after  it  has  done  its  share  toward 
insuring  the  perpetuation  of  its  species,  it  dies.  It  may 
meet  a  violent  death,  may  be  killed  by  accident  or  by.  ene- 
mies, before  the  life  cycle  is  completed.  And  this  is  the 
fate  of  the  vast  majority  of  animals  which  are  born  or 
hatched.  Or  death  may  come  before  the  time  for  birth  or 
hatching.  Of  the  millions  of  eggs  laid  by  a  fish,  each  egg 
a  new  fish  in  simplest  stage  of  development,  how  many  or 
rather  how  few  come  to  maturity,  how  few  complete  the 
cycle  of  life ! 

Of  death  we  know  the  essential  meaning.  Life  ceases 
and  can  never  be  renewed  in  the  body  of  the  dead  animal. 
It  is  important  that  we  include  the  words  "  can  never  be 
renewed,"  for  to  say  simply  that  "  life  ceases,"  that  is,  that 
the  performance  of  the  life  processes  or  functions  ceases, 
is  not  really  death.  It  is  easy  to  distinguish  in  most  cases 
between  life  and  death,  between  a  live  animal  and  a  dead 
one,  yet  there  are  cases  of  apparent  death  or  a  semblance  of 
death  which  are  very  puzzling.  The  test  of  life  is  usually 
taken  to  be  the  performance  of  life  functions,  the  assimila- 
tion of  food  and  excretion  of  waste,  the  breathing  in  of  oxy- 
gen, and  breathing  out  of  carbonic-acid  gas,  movement, 
feeling,  etc.  But  some  animals  can  actually  suspend  all 
of  these  functions,  or  at  least  reduce  them  to  such  a  mini- 
mum that  they  can  not  be  perceived  by  the  strictest  exami- 
nation, and  yet  not  be  dead.  That  is,  they  can  renew 
again  the  performance  of  the  life  processes.  Bears  and 
some  other  animals,  among  them  many  insects,  spend  the 
winter  in  a  state  of  death-like  sleep.  Perhaps  it  is  but  sleep ; 
and  yet  hibernating  insects  can  be  frozen  solid  and  remain 
frozen  for  weeks  and  months,  and  still  retain  the  power  of 
actively  living  again  in  the  following  spring.  Even  more 
remarkable  is  the  case  of  certain  minute  animals  called  Ro- 


104  ANIMAL  LIFE 

tatoria  and  of  others  called  Tardigrada,  or  bear-animalcules. 
These  bear-animalcules  live  in  water.  If  the  water  dries 
up,  the  animalcules  dry  up  too ;  they  shrivel  up  into  form- 
less little  masses  and  become  desiccated.  They  are  thus 
simply  dried-up  bits  of  organic  matter;  they  are  organic 
dust.  Now,  if  after  a  long  time — years  even — one  of  these 
organic  dust  particles,  one  of  these  dried-up  bear-animal- 
cules is  put  into  water,  a  strange  thing  happens.  The  body 
swells  and  stretches  out,  the  skin  becomes  smooth  instead 
of  all  wrinkled  and  folded,  and  the  legs  appear  in  normal 
shape.  The  body  is  again  as  it  was  years  before,  and  after 
a  quarter  of  an  hour  to  several  hours  (depending  on  the 
length  of  time  the  animal  has  lain  dormant  and  dried)  slow 
movements  of  the  body  parts  begin,  and  soon  the  animal- 
cule crawls  about,  begins  again  its  life  where  it  had  been 
interrupted.  Various  other  small  animals,  such  as  vinegar 
eels  and  certain  Protozoa,  show  similar  powers.  Certainly 
here  is  an  interesting  problem  in  life  and  death. 

When  death  comes  to  one  of  the  animals  with  which 
we  are  familiar,  we  are  accustomed  to  think  of  its  coming 
to  the  whole  body  at  some  exact  moment  of  time.  As  we 
stand  beside  a  pet  which  has  been  fatally  injured,  we  wait 
until  suddenly  we  say,  "  It  is  dead."  As  a  matter  of  fact, 
it  is  difficult  to  say  when  death  occurs.  Long  after  the 
heart  ceases  to  beat,  other  organs  of  the  body  are  alive — 
that  is,  are  able  to  perform  their  special  functions.  The 
muscles  can  contract  for  minutes  or  hours  (for  a  short  time 
in  warm-blooded,  for  a  long  time  in  cold-blooded  animals) 
after  the  animal  ceases  to  breathe  and  its  heart  to  beat. 
Even  longer  live  certain  cells  of  the  body,  especially  the 
amoeboid  white  blood-corpuscles.  These  cells,  very  like 
the  Ammba  in  character,  live  for 'days  after  the  animal  is, 
as  we  say,  dead.  The  cells  which  line  the  tracheal  tube 
leading  to  the  lungs  bear  cilia  or  fine  hairs  which  they 
wave  back  and  forth.  They  continue  this  movement  for 
days  after  the  heart  has  ceased  beating.  Among  cold- 


THE  LIFE  CYCLE 


105 


blooded  animals,  like  snakes  and  turtles,  complete  cessa- 
tion of  life  functions  comes  very  slowly,  even  after  the 
body  has  been  literally  cut  to  pieces. 

Thus  it  is  essential  in  defining  death  to  speak  of  a 
complete  and  permanent  cessation  of  the  performance  of 
the  life  processes. 


A  grasshopper  (Melanoplus  differentials)  killed  by  disease  caused  by 
parasitic  fungus.    On  golden-rod. 


CHAPTER  VI 

THE   PRIMARY   CONDITIONS   OF   ANIMAL   LIFE 

62.  Primary   conditions  and  special  conditions. — Certain 
primary  conditions  are  necessary  for  the  existence  of  all 
animals.     We  know  that  fishes  can  not  live  very  long  out 
of  water,  and  that  birds  can  not  live  in  water.     These, 
however,  are  special  conditions  which  depend  on  the  spe- 
cial structure  and  habits  of  these  two  particular  kinds  of 
backboned  animals.     But  the  necessity  of  a  constant  and 
sufficient  supply  of  air  is  a  necessity  common  to  both  ;  it  is 
one  of  the  primary  conditions  of  their  life.     All  animals 
must  have  air.     Similarly  both  fishes  and  birds,  and  all 
other  animals  as  well,  must  have  food.     This  is  another  one 
of  the  primary  conditions  of  animal  life.     That  backboned 
animals  must  find  somehow  a  supply  of  salts  or  compounds 
of  lime  to  form  into  bones  is  a  special  condition  peculiar 
to  these  animals.     Other  animals  having  shells  or  teeth 
composed  of  carbonate  or  phosphate  of  lime  are  subject  to 
-the  same  special  demand,  but  many  animals  have  no  hard 
parts,  and  therefore  need  no  lime. 

63.  Food. — All  the  higher  plants,  those  that  are  green 
(chlorophyll-bearing),  can  make  their  living  substance  out 
of  inorganic  matter  alone — that  is,  use  inorganic  substances 
as  food.     But  animals  can  not  do  this.     They  must  have 
already  formed  organic  matter  for  food.     This  organic  mat- 
ter may  be  the  living  or  dead  tissues  of  plants,  or  the  living 
or  dead  tissues  of  animals.     For  the  life  of  animals  it  is 
necessary  that  other  organisms  live,  or  have  lived.     It  is 
this  need  which  primarily  distinguishes  an  animal  from  a 

106 


THE  PRIMARY  CONDITIONS  OF  ANIMAL  LIFE    107 

plant.  Animals  can  not  exist  without  plants.  The  plants 
furnish  all  animals  with  food,  either  directly  or  indirectly. 
The  amount  of  food  and  the  kinds  of  food  required  by 
various  kinds  of  animals  are  special  conditions  depending 
on  the  size,  the  degree  of  activity,  the  structural  character 
of  the  body,  etc.,  of  the  animal  in  question.  Those  which 
do  the  most  need  most.  Those  with  warmest  blood,  great- 
est activity,  and  most  rapid  change  of  tissues  are  most 
dependent  on  abundance,  regularity,  and  fitness  of  their 
food.  As  we  well  know,  an  animal  can  live  for  a  longer  or 
shorter  time  without  food.  Men  have  fasted  for  a  month, 
or  even  two  months.  Among  cold-blooded  animals,  like  the 
reptiles,  the  general  habit  of  food  taking  is  that  of  an  occa- 
sional gorging,  succeeded  by  a  long  period  of  abstinence. 
Many  of  the  lower  animals  can  go  without  food  for  surpris- 
ingly long  periods  without  loss  of  life.  But  the  continued 
lack  of  food  results  inevitably  in  death.  Any  animal  may 
be  starved  in  time. 

If  water  be  held  not  to  be  included  in  the  general  con- 
ception of  food,  then  special  mention  must  be  made  of  the 
necessity  of  water  as  one  of  the  primary  conditions  of  ani- 
mal life.  Protoplasm,  the  basis  of  life,  is  a  fluid,  although 
thick  and  viscous.  To  be  fluid  its  components  must  be 
dissolved  or  suspended  in  water.  In  fact,  all  the  truly 
living  substance  in  an  animal's  body  contains  water.  The 
water  necessary  for  the  animal  may  be  derived  from  the 
other  food,  all  of  which  contains  water  in  greater  or 
less  quantity,  or  may  be  taken  apart  from  the  other 
food,  by  drinking  or  by  absorption  through  the  skin. 
Sheep  are  seldom  seen  to  drink,  for  they  find  almost 
enough  water  in  their  green  food.  Fur  seals  never  drink, 
for  they  absorb  the  water  needed  through  pores  in  the 
skin. 

64.  Oxygen. — Animals  must  have  air  in  order  to  live, 
but  the  essential  element  of  the  air  which  they  need  is  its 
oxygen.  For  the  metabolism  of  the  body,  for  the  chemical 


108  ANIMAL   LIFE 

changes  which  take  place  in  the  body  of  every  living  ani- 
mal, a  supply  of  oxygen  is  required.  This  oxygen  is  de- 
rived directly  or  indirectly  from  the  air.  The  atmosphere 
of  the  earth  is  composed  of  79.02  parts  of  nitrogen  (includ- 
ing argon),  .03  parts  of  carbonic  acid,  and  20.95  parts  of 
oxygen.  Thus  all  the  animals  which  live  on  land  are  en- 
veloped by  a  substance  containing  nearly  21  per  cent  of 
oxygen.  But  animals  can  live  in  an  atmosphere  containing 
much  less  oxygen.  Certain  mammals,  experimented  on, 
lived  without  difficulty  in  an  atmosphere  containing  only 
14  per  cent  of  oxygen  ;  when  the  oxygen  was  reduced  to  7 
per  cent  serious  disturbances  were  caused  in  the  animal's 
condition,  and  death  by  suffocation  ensued  when  3  per 
cent  of  oxygen  was  left  in  the  atmosphere.  Animals  which 
live  in  water  get  their  oxygen,  not  from  the  water  itself 
(water  being  composed  of  hydrogen  and  oxygen),  but  from 
air  which  is  mechanically  mixed  with  the  water.  Fishes 
breathe  the  air  which  is  mixed  with  or  dissolved  in  the 
water.  This  scanty  supply  therefore  constitutes  their  at- 
mosphere, for  in  water  from  which  all  air  is  excluded  no 
animal  can  breathe.  Whatever  the  habits  of  life  of  the 
animal,  whether  it  lives  on  the  land,  in  the  ground,  or  in 
the  water,  it  must  have  oxygen  or  die. 

65.  Temperature,  pressure,  and  other  conditions. — Some 
physiologists  include  among  the  primary  or  essential  gen- 
eral conditions  of  animal  life  such  conditions  as  favorable 
temperature  and  favorable  pressure.  It  is  known  from  ob- 
servation and  experiment  that  animals  die  when  a  too  low 
or  a  too  high  temperature  prevails.  The  minimum  or 
maximum  of  temperature  between  which  limits  an  animal 
can  live  varies  much  among  different  kinds  of  animals.  It 
is  familiar  knowledge  that  many  kinds  of  animals  can  be 
frozen  and  yet  not  be  killed.  Insects  and  other  small  ani- 
mals may  lie  frozen  through  a  winter  and  resume  active 
life  again  in  the  spring.  An  experimenter  kept  certain 
fish  frozen  in  blocks  of  ice  at  a  temperature  of  —15°  C. 


THE  PRIMARY  CONDITIONS  OF  ANIMAL  LIFE    109 

for  some  time  and  then  gradually  thawed  them  out  un- 
hurt. Only  very  hardy  kinds  adapted  to  the  cold  would, 
however,  survive  such  treatment.  There  is  no  doubt  that 
every  part  of  the  body,  all  of  the  living  substance,  of  these 
fish  was  frozen,  for  specimens  at  this  temperature  could  be 
broken  and  pounded  up  into  fine  ice  powder.  But  a  tem- 
perature of  —20°  C.  killed  the  fish.  Frogs  lived  after  being 
kept  at  a  temperature  of  —28°  C.,  centipeds  at  —50°  C.,and 
certain  snails  endured  a  temperature  of  —120°  C.  without 
dying.  At  the  other  extreme,  instances  are  known  of  ani- 
mals living  in  water  (hot  springs  or  water  gradually  heated 
with  the  organisms  in  it)  of  a  temperature  as  high  as  50°  C. 
Experiments  with  Ammbce  show  that  these  simplest  animals 
contract  and  cease  active  motion  at  35°  C.,  but  are  not  killed 
until  a  temperature  of  40°  to  45°  C.  is  reached.  The  little 
fish  called  blob  or  miller's  thumb  ( Coitus  ictalops)  has  been 
seen  lying  boiled  in  the  bottom  of  the  hot  springs  in  the 
Yellowstone  Park ;  but  it  must  have  entered  these  springs 
through  streams  of  a  temperature  little  below  the  boiling 
point. 

The  pressure  or  weight  of  the  atmosphere  on  the  sur- 
face of  the  earth  is  nearly  fifteen  pounds  on  each  square 
inch.  This  pressure  is  exerted  equally  in  all  directions,  so 
that  an  object  on  the  earth's  surface  sustains  a  pressure  on 
each  square  inch  of  its  surface  exposed  to  the  air  of  fifteen 
pounds.  Thus  all  animals  living  on  the  earth's  surface  or 
near  it  live  under  this  pressure,  and  know  no  other  condi- 
tion. For  this  reason  they  do  not  notice  it.  The  animals 
that  live  in  water,  however,  sustain  a  much  greater  pres- 
sure, this  pressure  increasing  with  the  depth.  Certain 
ocean  fishes  live  habitually  at  great  depths,  as  two  to  five 
miles,  where  the  pressure  is  equivalent  to  that  of  many 
hundred  atmospheres.  If  these  fishes  are  brought  to  the 
surface  their  eyes  bulge  out  fearfully,  being  pushed  out 
through  reduced  expansion ;  their  scales  fall  off  because  of 
the  great  expansion  of  the  skin,  and  the  stomach  is  pushed 


HO  ANIMAL  LIFE 

out  from  the  mouth  till  it  is  wrong  side  out.  Indeed,  the 
bodies  sometimes  burst.  Their  bodies  are  accustomed  to 
this  great  pressure,  and  when  this  outside  pressure  is  sud- 
denly removed  the  body  may  be  bursted.  Sometimes 
such  a  fish  is  raised  from  its  proper  level  by  a  struggle 
with  its  prey,  when  both  captor  and  victim  may  be  de- 
stroyed by  the  expansion  of  the  body.  Some  fishes  die  on 
being  taken  out  of  water  through  the  swelling  of  the  air 
bladder  and  the  bursting  of  its  blood-vessels.  If  an  animal 
which  lives  normally  on  the  surface  of  the  earth  is  taken 
up  a  very  high  mountain  or  is  carried  up  in  a  balloon  to  a 
great  altitude  where  the  pressure  of  the  atmosphere  is 
much  less  than  it  is  at  the  earth's  surface,  serious  conse- 
quences may  ensue,  and  if  too  high  an  altitude  is  reached 
death  occurs.  This  death  may  be  in  part  due  to  the  diffi- 
culty in  breathing  in  sufficient  oxygen  to  maintain  life,  but 
it  is  probably  chiefly  due  to  disturbances  caused  by  the 
removal  of  the  pressure  to  which  the  body  is  accustomed 
and  is  structurally  adapted  to  withstand.  A  famous  bal- 
loon ascension  was  made  in  Paris  in  1875  by  three  men. 
After  the  balloon  had  reached  a  height  of  nearly  24,000 
feet  (almost  five  miles)  the  men  began  to  lose  conscious- 
ness. On  the  sinking  of  the  balloon  to  about  20,000  feet 
the  men  regained  consciousness  again  and  threw  out  bal- 
last so  that  the  balloon  rose  to  a  height  of  over  25,000  feet. 
This  time  all  three  became  wholly  unconscious,  and  on  the 
balloon  sinking  again  only  one  regained  consciousness. 
The  other  two  died  in  the  foolhardy  experiment.  All  liv- 
ing animals  are  accustomed  to  live  under  a  certain  pres- 
sure, and  there  are  evidently  limits  of  maximum  or  mini- 
mum pressure  beyond  which  no  animal  at  present  existing 
can  go  and  remain  alive. 

But  in  the  case  both  of  temperature  and  pressure  con- 
ditions it  is  easy  to  conceive  that  animals  might  exist  which 
could  live  under  temperature  and  pressure  conditions  not 
included  between  the  minimum  and  maximum  limits  of  each 


THE  PRIMARY  CONDITIONS  OF  ANIMAL  LIFE    HI 

as  determined  by  animals  so  existing.  But  it  is  impossible 
to  conceive  of  animals  which  could  live  without  oxygen  or 
without  organic  food.  The  necessities  of  oxygen  and  organic 
food  (and  water)  are  the  primary  or  essential  conditions 
for  the  existence  of  any  animals. 

Of  course,  we  might  include  such  conditions,  among 
the  primary  conditions,  as  the  light  and  heat  of  the  sun, 
the  action  of  gravitation,  and  olher  physical  conditions 
without  which  existence  or.  life  of  any  kind  would  be  im- 
possible on  this  earth.  But  we  here  consider  by  "  primary 
conditions  of  animal  life  "  rather  those  necessities  of  living 
animals  as  opposed  to  the  necessities  of  living  plants. 
Neither  animals  nor  plants  could  exist  without  the  sun, 
whence  they  derive  directly  or  indirectly  all  their  energy. 

66.  Difference  between  animals  and  plants. — It  is  easy  to 
distinguish  between  the  animal  and  plant  when  a  butterfly 
is  fluttering  about  a  blossoming  cherry  tree  or  a  cow  feed- 
ing in  a  field  of  clover.  It  is  not  so  easy,  if  it  is,  indeed, 
possible,  to  say  which  is  plant  and  which  is  animal  when 
the  simplest  plants  are  compared  with  the  simplest  ani- 
mals. It  is  almost  impossible  to  so  define  animals  as  to 
distinguish  all  of  them  from  all  plants,  or  so  to  define 
plants  as  to  distinguish  all  of  them  from  all  animals. 
While  most  animals  have  the  power  of  locomotion,  some, 
like  the  sponges  and  polyps  and  barnacles  and  numerous 
parasites,  are  fixed.  While  most  plants  are  fixed,  some  of 
the  low  aquatic  forms  have  the  po  sver  of  spontaneous  loco- 
motion, and  all  plants  have  some  power  of  motion,  as  espe- 
cially exemplified  in  the  revolution  of  the  apex  of  the 
growing  stem  and  root,  and  the  spiral  twisting  of  tendrils, 
and  in  the  sudden  closing  of  the  leaves  of  the  sensitive 
plant  when  touched.  Among  the  green  or  chlorophyll- 
bearing  plants  the  food  consists  chiefly  of  inorganic  sub- 
stances, especially  of  carbon  which  is  taken  from  the  car- 
bonic-acid gas  in  the  atmosphere,  and  of  water.  But  some 
green-leaved  plants  feed  also  in  part  on  organic  food. 


112  ANIMAL   LIFE 

Such  are  the  pitcher-plants  and  sun-dews,  and  Venus-fly- 
traps, which  catch  insects  and  use  them  for  food  nutrition. 
But  there  are  many  plants,  the  fungi,  which  are  not  green 
—that  is,  which  do  not  possess  chlorophyll,  the  substance 
on  which  seems  to  depend  the  power  to  make  organic 
matter  out  of  inorganic  substances.  These  plants  feed  on 
organic  matter  as  animals  do.  The  cells  of  plants  (in  their 
young  stages,  at  least)  have  a  wall  composed  of  a  peculiar 
carbohydrate  substance  called,  cellulose,  and  this  cellulose 
was  for  a  long  time  believed  not  to  occur  in  the  body  of 
animals.  But  now  it  is  known  that  certain  sea-squirts 
(Tunicata)  possess  cellulose.  It  is  impossible  to  find  any 
set  of  characteristics,  or  even  any  one  characteristic,  which 
is  possessed  only  by  plants  or  only  by  animals.  But  nearly 
all  of  the  many-celled  plants  and  animals  may  be  easily 
distinguished  by  their  general  characteristics.  The  power 
of  breaking  up  carbonic-acid  gas  into  carbon  and  oxygen 
and  assimilating  the  carbon  thus  obtained,  the  presence  of 
chlorophyll,  and  the  cell  walls  formed  of  cellulose,  are  char- 
acteristics constant  in  all  typical  plants.  In  addition,  the 
fixed  life  of  plants,  and  their  general  use  of  inorganic  sub- 
stances for  food  instead  of  organic,  are  characteristics 
readily  observed  and  practically  characteristic  of  many- 
celled  plants.  When  the  thousands  of  kinds  of  one-celled 
organisms  are  compared,  however,  it  is  often  a  matter  of 
great  difficulty  or  of  real  impossibility  to  say  whether  a 
given  organism  should  be  assigned  to  the  plant  kingdom 
or  to  the  animal  kingdom.  In  general  the  distinctive 
characters  of  plants  are  grouped  around  the  loss  of  the 
power  of  locomotion  and  related  to  or  dependent  upon  it. 

67.  Living  organic  matter  and  inorganic  matter, — It  would 
seem  to  be  an  easy  matter  to  distinguish  an  organism — that 
is,  a  living  animal  or  plant— from  an  inorganic  substance.  It 
is  easy  to  distinguish  a  dove  or  a  sunflower  from  stone,  and 
practically  there  never  is  any  difficulty  in  making  such  dis- 
tinctions. But  when  we  try  to  define  living  organic  matter, 


THE  PRIMARY   CONDITIONS  OF  ANIMAL  LIFE     H3 

and  to  describe  those  characteristics  which  are  peculiar  to 
it,  which  absolutely  distinguish  it  from  inorganic  matter, 
we  meet  with  some  difficulties.  At  least  many  of  the  char- 
acteristics commonly  ascribed  to  organisms,  as  peculiar  to 
them,  are  not  so.  The  possession  of  organs,  or  the  composi- 
tion of  the  body  of  distinct  parts,  each  with  a  distinct  func- 
tion, but  all  working  together,  and  depending  on  each  other, 
is  as  true  of  a  steam-engine  as  of  a  horse.  That  the  work 
done  by  the  steam-engine  depends  upon  fuel  is  true ;  but 
so  it  is  that  the  work  done  by  the  horse  depends  upon  fuel, 
or  food  as  we  call  it  in  the  case  of  the  animal.  The  oxida- 
tion or  burning  of  this  fuel  in  the  engine  is  wholly  compar- 
able with  the  oxidation  of  the  food,  or  the  muscle  and  fat 
it  is  turned  into,  in  the  horse's  body.  The  composition  of 
the  bodies  of  animals  and  plants  of  tiny  structural  units, 
the  cells,  is  in  many  ways  comparable  with  the  composition 
of  some  rocks  of  tiny  structural  units,  the  crystals.  But 
not  to  carry  such  rather  quibbling  comparisons  too  far,  it 
may  be  said  that  organisms  are  distinguished  from  organic 
substances  by  the  following  characteristics  :  Organization ; 
the  power  to  make  over  inorganic  substances  into  organic 
matter,  or  the  changing  of  organic  matter  of  one  kind,  as 
plant  matter,  into  another  kind,  as  animal  matter ;  motion, 
the  power  of  spontaneous  movement  in  response  to  stimuli ; 
sensation,  the  power  of  being  sensible  of  external  stimuli ; 
reproduction,  the  power  of  producing  new  beings  like  them- 
selves ;  and  adaptation,  the  power  of  responding  to  external 
conditions  in  a  way  useful  to  the  organism.  Through  adap- 
tation organisms  continue  to  exist  despite  the  changing  of 
conditions.  If  the  conditions  surrounding  an  inorganic 
body  change,  even  gradually,  the  inorganic  body  does  not 
change  to  adapt  itself  to  these  conditions,  but  resists  them 
until  no  longer  able  to  do  so,  when  it  loses  its  identity  or 
integrity. 


CHAPTER  VII 

THE    CROWD    OF   ANIMALS   AND   THE   STRUGGLE    FOR 
EXISTENCE 

68.  The  crowd  of  animals. — All  animals  feed  upon  living 
organisms,  or  on  their  dead  bodies.  Hence  each  animal 
throughout  its  life  is  busy  with  the  destruction  of  other 
organisms,  or  with  their  removal  after  death.  If  those 
creatures  upon  which  others  feed  are  to  hold  their  own,  there 
must  be  enough  born  or  developed  to  make  good  the  drain 
upon  their  numbers.  If  the  plants  did  not  fill  up  their 
ranks  and  make  good  their  losses,  the  animals  that  feed 
on  them  would  perish.  If  the  plant-eating  animals  were 
destroyed,  the  flesh-eating  animals  would  in  turn  disappear. 
But,  fortunately,  there  is  a  vast  excess  in  the  process  of 
reproduction.  More  plants  sprout  than  can  find  room  to 
grow.  More  animals  are  born  than  can  possibly  survive. 
The  process  of  increase  among  animals  is  correctly  spoken 
of  as  multiplication.  Each  species  tends  to  increase  in 
geometric  ratio,  but  as  it  multiplies  its  members  it  finds 
the  world  already  crowded  with  other  species  doing  the 
same  thing.  A  single  pair  of  any  species  whatsoever,  if  not 
restrained  by  adverse  conditions,  would  soon  increase  to 
such  an  extent  as  to  fill  the  whole  world  with  its  progeny. 
An  annual  plant  producing  two  seeds  only  would  have 
1,048,576  descendants  at  the  end  of  twenty-one  years,  if 
each  seed  sprouted  and  matured.  The  ratio  of  increase  is 
therefore  a  matter  of  minor  importance.  It  is  the  ratio  of 
net  increase  above  loss  which  determines  the  fate  of  a  spe- 
cies. Those  species  increase  in  numbers  whose  gain  exceeds 
114 


THE  STRUGGLE  FOR  EXISTENCE  H5 

the  death  rate,  and  those  which  "  live  beyond  their  means  " 
must  sooner  or  later  disappear.  One  of  the  most  abundant 
of  birds  is  the  fulmar  petrel,  which  lays  but  one  egg  yearly. 
It  has  but  few  enemies,  and  this  low  rate  of  increase  suf- 
fices to  cover  the  seas  within  its  range  with  petrels. 

It  is  difficult  to  realize  the  inordinate  numbers  in  which 
each  species  would  exist  were  it  not  for  the  checks  produced 
by  the  presence  of  other  animals.  Certain  Protozoa  at  their 
normal  rate  of  increase,  if  none  were  devoured  or  destroyed, 
might  fill  the  entire  ocean  in  about  a  week.  The  conger- 
eel  lays,  it  is  said,  15,000,000  eggs.  If  each  egg  grew 
up  to  maturity  and  reproduced  itself  in  the  same  way  in 
less  than  ten  years  the  sea  would  be  solidly  full  of  conger- 
eels.  If  the  eggs  of  a  common  house-fly  should  develop,  and 
each  of  its  progeny  should  find  the  food  and  temperature  it 
needed,  with  no  loss  and  no  destruction,  the  people  of  a  city  in 
which  this  might  happen  could  not  get  away  soon  enough  to 
escape  suffocation  from  a  plague  of  flies.  Whenever  any  in- 
sect is  able  to  develop  a  large  percentage  of  the  eggs  laid,  it 
becomes  at  once  a  plague.  Thus  originate  plagues  of  grass- 
hoppers, locusts,  and  caterpillars.  But  the  crowd  of  life  is 
such  that  no  great  danger  exists.  The  scavenger  destroys 
the  decaying  flesh  where  the  fly  would  lay  its  eggs.  Minute 
creatures,  insects,  bacteria,  Protozoa  are  parasitic  within 
the  larva  and  kill  it.  Millions  of  flies  perish  for  want  of 
food.  Millions  more  are  destroyed  by  insectivorous  birds, 
and  millions  are  slain  by  parasites.  The  final  result  is  that 
from  year  to  year  the  number  of  flies  does  not  increase. 
Linnaeus  once  said  that  "  three  flies  would  devour  a  dead 
horse  as  quickly  as  a  lion."  Equally  soon  would  it  be  de- 
voured by  three  bacteria,  for  the  decay  of  the  horse  is  due 
to  the  decomposition  of  its  flesh  by  these  microscopic  plants 
which  feed  upon  it.  "  Even  slow-breeding  man,"  says  Dar- 
win, "  has  doubled  in  twenty-five  years.  At  this  rate  in  less 
than  a  thousand  years  there  would  literally  not  be  standing 
room  for  his  progeny.  The  elephant  is  reckoned  the  slow- 


ANIMAL  LIFE 

est  breeder  of  all  known  animals.  It  begins  breeding  when 
thirty  years  old  and  goes  on  breeding  until  ninety  years 
old,  bringing  forth  six  young  in  the  interval,  and  surviving 
till  a  hundred  years  old.  If  this  be  so,  after  about  eight 
hundred  years  there  would  be  19,000,000  elephants  alive, 
descended  from  the  first  pair."  A  few  years  more  of  the 
unchecked  multiplication  of  the  elephant  and  every  foot  of 
land  on  the  earth  would  be  covered  by  them. 

Yet  the  number  of  elephants  does  not  increase.  In  gen- 
eral, the  numbers  of  every  species  of  animal  in  the  state  of 
Nature  remain  about  stationary.  Under  the  influence  of 
man  most  of  them  slowly  diminish.  There  are  about  as 
many  squirrels  in  the  forest  one  year  as  another,  about  as 
many  butterflies  in  the  field,  about  as  many  frogs  in  the 
pond.  Wolves,  bears,  deer,  wild  ducks,  singing  birds,  fishes, 
tend  to  grow  fewer  and  fewer  in  inhabited  regions,  because 
the  losses  from  the  hand  of  man  are  added  to  the  losses  in 
the  state  of  Nature. 

It  has  been  shown  that  at  the  normal  rate  in  increase  of 
English  sparrows,  if  none  were  to  die  save  of  old  age,  it 
would  take  but  twenty  years  to  give  one  sparrow  to  every 
square  inch  in  the  State  of  Indiana.  Such  an  increase  is 
actually  impossible,  for  more  than  a  hundred  other  species 
of  similar  birds  are  disputing  the  same  territory  with  the 
power  of  increase  at  a  similar  rate.  There  can  not  be  food 
and  space  for  all.  With  such  conditions  a  struggle  is  set 
up  between  sparrow  and  sparrow,  between  sparrow  and 
other  birds,  and  between  sparrow  and  the  conditions  of  life. 
Such  a  conflict  is  known  as  the  struggle  for  existence. 

69.  The  struggle  for  existence.— The  struggle  for  exist- 
ence is  threefold:  (a)  among  individuals  of  one  species, 
as  sparrow  and  sparrow;  (#)  between  individuals  of  differ- 
ent species,  as  sparrow  with  bluebird  or  robin  ;  and  (c)  with 
the  conditions  of  life,  as  the  effort  of  the  sparrow  to  keep 
warm  in  winter  and  to  find  water  in  summer.  All  three 
forms  of  this  struggle  are  constantly  operative  and  with 


THE  STRUGGLE  FOR  EXISTENCE  H7 

every  species.  In  some  regions  the  one  phase  may  be  more 
destructive,  in  others  another.  Where  the  conditions  of 
life  are  most  easy,  as  in  the  tropics,  the  struggle  of  species 
with  species,  of  individual  with  individual,  is  the  most 
severe. 

No  living  being  can  escape  from  any  of  these  three 
phases  of  the  struggle  for  existence.  For  reasons  which  we 
shall  see  later,  it  is  not  well  that  any  should  escape,  for  "  the 
sheltered  life,"  the  life  withdrawn  from  the  stress  of  effort, 
brings  the  tendency  to  degeneration. 

Because  of  the  destruction  resulting  from  the  struggle 
for  existence,  more  of  every  species  are  born  than  can 
possibly  find  space  or  food  to  mature.  The  majority  fail 
to  reach  their  full  growth  because,  for  one  reason  or  an- 
other, they  can  not  do  so.  All  live  who  can.  Each  strives 
to  feed  itself,  to  save  its  own  life,  to  protect  its  young. 
But  with  all  their  efforts  only  a  portion  of  each  species 
succeed. 

70.  Selection  by  Nature. — But  the  destruction  in  Nature 
is  not  indiscriminate.  In  the  long  run  those  least  fitted  to 
resist  attack  are  the  first  to  perish.  It  is  the  slowest  ani- 
mal which  is  soonest  overtaken  by  those  which  feed  upon 
it.  It  is  the  weakest  which  is  crowded  away  from  the  feed- 
ing-place by  its  associates.  It  is  the  least  adapted  which  is 
first  destroyed  by  extremes  of  heat  and  cold.  Just  as  a 
farmer  improves  his  herd  of  cattle  by  destroying  his  weak- 
est or  roughest  calves,  reserving  the  strong  and  fit  for  par- 
entage, so,  on  an  inconceivably  large  scale,  the  forces  of 
Nature  are  at  work  purifying,  strengthening,  and  fitting  to 
their  surroundings  the  various  species  of  animals.  This 
process  has  been  called  natural  selection,  or  the  survival  of 
the  fittest.  But  by  fittest  in  this  sense  we  mean  only  best 
adapted  to  the  surroundings,  for  this  process,  like  others  in 
Nature,  has  itself  no  necessarily  moral  element.  The  song- 
bird becomes  through  this  process  more  fit  for  the  song-bird 
life,  the  hawk  becomes  more  capable  of  killing  and  tear- 


ANIMAL  LIFE 

ing,  and  the  woodpecker  better  fitted  to  extract  grubs  from 
the  tree. 

In  the  struggle  of  species  with  species  one  may  gain  a 
little  one  year  and  another  the  next,  the  numbers  of  each 
species  fluctuating  a  little  with  varying  circumstances,  but 
after  a  time,  unless  disturbed  by  the  hand  of  man,  a  point 
will  be  reached  when  the  loss  will  almost  exactly  balance 
the  increase.  This  produces  a  condition  of  apparent  equi- 
librium. The  equilibrium  is  broken  when  any  individual  or 
group  of  individuals  becomes  capable  of  doing  something 
more  than  hold  its  own  in  the  struggle  for  existence. 

When  the  conditions  of  life  become  adverse  to  the  exist- 
ence of  a  species  it  has  three  alternatives,  or,  better,  one  of 
three  things  happens,  namely,  migration,  adaptation,  extinc- 
tion. The  migration  of  birds  and  some  other  animals  is  a 
systematic  changing  of  environment  when  conditions  are 
unfavorable  to  life.  When  the  snow  and  ice  come,  the  fur- 
seal  forsakes  the  islands  on  which  it  breeds,  and  which  are 
its  real  home,  and  spends  the  rest  of  the  year  in  the  open 
sea,  returning  at  the  close  of  winter.  Some  other  animals 
migrate  irregularly,  removing  from  place  to  place  as  condi- 
tions become  severe  or  undesirable.  The  Rocky  Mountain 
locusts,  which  breed  on  the  great  plateau  along  the  eastern 
base  of  the  Eocky  Mountains,  sometimes  increase  so  rapidly 
in  numbers  that  they  can  not  find  enough  food  in  the  scanty 
vegetation  of  this  region.  Then  great  hosts  of  them  fly 
high  into  the  air  until  they  meet  an  air  current  moving 
toward  the  southeast.  The  locusts  are  borne  by  this  cur- 
rent or  wind  hundreds  of  miles,  until,  when  they  come  to 
the  great  grain-growing  Mississippi  Valley,  they  descend 
and  feed  to  their  hearts'  content,  and  to  the  dismay  of  the 
Nebraska  and  Kansas  farmer.  These  great  forced  migra- 
tions used  to  occur  only  too  often,  but  none  has  taken  place 
since  1878,  and  it  is  probable  that  none  will  ever  occur 
again.  With  the  settlement  of  the  Eocky  Mountain  plateau 
by  farmers,  food  is  plenty  at  home.  And  the  constant  fight- 


THE  STRUGGLE  FOR  EXISTENCE  H9 

ing  of  the  locusts  by  the  farmers,  by  plowing  up  their  eggs, 
and  crushing  and  burning  the  young  hoppers,  keeps  down 
their  numbers. 

Another  animal  of  interesting  migratory  habits  is  the 
lemming,  a  mouse-like  animal  nearly  as  large  as  a  rat,  which 
lives  in  the  arctic  regions.  At  intervals  varying  from  five 
to  twenty  years  the  cultivated  lands  of  Norway  and  Sweden, 
where  the  lemming  is  ordinarily  unknown,  are  overrun  by 
vast  numbers  of  these  little  animals.  They  come  as  an 
army,  steadily  and  slowly  advancing,  always  in  the  same 
direction,  and  "  regardless  of  all  obstacles,  swimming  across 
streams  and  even  lakes  of  several  miles  in  breadth,  and 
committing  considerable  devastation  on  their  line  of  march 
by  the  quantity  of  food  they  consume.  In  their  turn  they 
are  pursued  and  harassed  by  crowds  of  beasts  and  birds  of 
prey,  as  bears,  wolves,  foxes,  dogs,  wild  cats,  stoats,  weasels, 
eagles,  hawks,  and  owls,  and  never  spared  by  man ;  even 
the  domestic  animals  not  usually  predaceous,  as  cattle, 
foals,  and  reindeer,  are  said  to  join  in  the  destruction, 
stamping  them  to  the  ground  with  their  feet  and  even  eat- 
ing their  bodies.  Numbers  also  die  from  disease  apparently 
produced  from  overcrowding.  None  ever  return  by  the 
course  by  which  they  came,  and  the  onward  march  of  the 
survivors  never  ceases  until  they  reach  the  sea,  into  which 
they  plunge,  and  swimming  onward  in  the  same  direction 
as  before  perish  in  the  waves."  One  of  these  great  migra- 
tions lasts  for  from  one  to  three  years.  But  it  always  ends 
in  the  total  destruction  of  the  migrating  army.  But  the 
migration  may  be  of  advantage  to  the  lemmings  which  re- 
main in  the  original  breeding  grounds,  leaving  them  with 
enough  food,  so  that,  on  the  whole,  the  migration  results  in 
gain  to  the  species. 

But  most  animals  can  not  migrate  to  their  betterment. 
In  that  case  the  only  alternatives  are  adaptation  or  destruc- 
tion. Some  individuals  by  the  possession  of  slight  advan- 
tageous variations  of  structure  are  able  to  meet  the  new 


120  ANIMAL  LIFE 

demands  and  survive,  the  rest  die.  The  survivors  produce 
young  similarly  advantageously  different  from  the  general 
type,  and  the  adaptation  increases  with  successive  genera- 
tions. 

71.  Adjustment  to  surroundings  a  result  of  natural  selec- 
tion.— To  such  causes  as  these  we  must  ascribe  the  nice 
adjustment  of  each  species  to  its  surroundings.     If  a  species 
or  a  group  of  individuals  can  not  adapt  itself  to  its  environ- 
ment, it  will  be  crowded  out  by  others  that  can  do  so.     The 
former  will  disappear  entirely  from  the  earth,  or  else  will  be 
limited  to  surroundings  with  which  it  comes  into  perfect 
adjustment.     A  partial  adjustment  must  with  time  become 
a  complete  one,  for  the  individuals  not  adapted  will  be 
exterminated  in  the  struggle  for  life.     In  this  regard  very 
small  variations  may  lead  to  great  results.     A  side  issue 
apparently  of  little  consequence  may  determine  the  fate  of 
a  species.     Any  advantage,  no  matter  how  small,  will  turn 
the  scale  of  life  in  favor  of  its  possessor  and  his  progeny. 
"Battle  within  battle,"  says  a  famous  naturalist,  "must  be 
continually  recurring,  with  varying  success.     Yet  in  the 
long  run  the  forces  are  so  nicely  balanced  that  the  face  of 
Nature  remains  for  a  long  time  uniform,  though  assuredly 
the  merest  trifle  would  give  the  victory  to  one  organic  being 
over  another." 

72.  Artificial  selection. — It  has  been  long  known  that  the 
nature  of  a  herd  or  race  of  animals  can  be  materially  altered 
by  a  conscious  selection  on  the  part  of  man  of  these  indi- 
viduals which  are  to  become  parents.     To  "  weed  out "  a 
herd  artificially  is  to  improve  its  blood.     To  select  for  re- 
production the  swiftest  horses,  the  best  milk  cows,  the  most 
intelligent  dogs,  is  to  raise  the  standard  of  the  herd  or 
race  in  each  of  these  respects  by  the  simple  action  of  hered- 
ity.    Artificial  selection  has  been  called  the  "magician's 
wand,"  by  which  the  breeder  can  summon  up  whatever 
animal  form  he  will.     If  the  parentage  is  chosen  to  a  defi- 
nite end,  the  process  of  heredity  will  develop  the  form 


THE  STRUGGLE  FOR  EXISTENCE  121 

desired  by  a  force  as  unchanging  as  that  by  which  a  stream 
turns  a  mill. 

From  the  wild  animals  about  him  man  has  developed 
the  domestic  animals  which  he  finds  useful.  The  dog 
which  man  trains  to  care  for  his  sheep  is  developed  by 
selection  from  the  most  tractable  progeny  of  the  wolf  which 
once  devoured  his  flocks.  By  the  process  of  artificial  selec- 
tion those  individuals  that  are  not  useful  to  man  or  pleas- 
ing to  his  fancy  have  been  destroyed,  and  those  which  con- 
tribute to  his  pleasure  or  welfare  have  been  preserved  and 
allowed  to  reproduce  their  kind.  The  various  fancy  breeds 
of  pigeons — the  carriers,  pouters,  tumblers,  ruff-necks,  and 
fan-tails — are  all  the  descendants  of  the  wild  dove  of  Eu- 
rope (Columba  livia).  These  breeds  or  races  or  varieties 
have  been  produced  by  artificial  selection.  So  it  is  with 
the  various  breeds  of  cattle  and  of  hogs  and  of  horses 
and  dogs. 

In  this  artificial  selection  new  variations  are  more  rap- 
idly produced  than  in  Nature  by  means  of  intercrossing 
different  races,  and  by  a  more  rapid  weeding  out  of  un- 
favorable— that  is,  of  undesirable — variations.  The  rapid 
production  of  variations  and  the  careful  preservation  of 
the  desirable  ones  and  rigid  destruction  of  undesirable 
ones  are  the  means  by  which  many  races  of  domestic  ani- 
mals are  produced.  This  is  artificial  selection. 

73.  Dependence  of  species  on  species.— There  was  intro- 
duced into  California  from  Australia,  on  young  orange  trees, 
a  few  years  ago,  an  insect  pest  called  the  cottony  cushion 
scale  (Iccrya  purchasi).  This  pest  increased  in  numbers 
with  extraordinary  rapidity,  and  in  four  or  five  years  threat- 
ened to  destroy  completely  the  great  orange  orchards  of 
California.  Artificial  remedies  were  of  little  avail.  Finally, 
an  entomologist  was  sent  to  Australia'  to  find  out  if  this 
scale  insect  had  not  some  special  natural  enemy  in  its 
native  country.  It  was  found  that  in  Australia  a  certain 
species  of  lady-bird  beetle  attacked  and  fed  on  the  cottony 


122  ANIMAL  LIFE 

cushion  scales  and  kept  them  in  check.  Some  of  these 
lady-birds  ( Vedalia  cardinalis)  were  brought  to  California 
and  released  in  .a  scale-infested  orchard.  The  lady-birds, 
having  plenty  of  food,  thrived  and  produced  many  young. 
Soon  the  lady-birds  were  in  such  numbers  that  numbers  of 
them  could  be  distributed  to  other  orchards.  In  two  or 
three  years  the  Vedalias  had  become  so  numerous  and 
widely  distributed  that  the  cottony  cushion  scales  began  to 
diminish  perceptibly,  and  soon  the  pest  was  nearly  wiped 
out.  But  with  the  disappearance  of  the  scales  came  also  a 
disappearance  of  the  lady-birds,  and  it  was  then  discovered 
that  the  Vedalias  fed  only  on  cottony  cushion  scales  and 
could  not  live  where  the  scales  were  not.  So  now,  in  order 
to  have  a  stock  of  Vedalias  on  hand  in  California  it  is  neces- 
sary to  keep  protected  some  colonies  of  the  cottony  cushion 
scale  to  serve  as  food.  Of  course,  with  the  disappearance 
of  the  predaceous  lady-birds  the  scale  began  to  increase 
again  in  various  parts  of  the  State,  but  with  the  sending  of 
Vedalias  to  these  localities  the  scale  was  again  crushed. 
How  close  is  the  interdependence  of  these  two  species ! 

Similar  relations  can  be  traced  in  every  group  of  ani- 
mals. When  the  salmon  cease  to  run  in  the  Sacramento 
River  in  California  the  otter  which  feeds  on  them  takes,  it 
is  said,  to  robbing  the  poultry-yards ;  and  the  bear,  which 
also  feeds  on  fish,  strikes  out  for  other  game,  taking  fruit 
or  chickens  or  bee-hives,  whatever  he  may  find. 


CHAPTER  VIII 

ADAPTATIONS 

74.  Origin  of  adaptations. — The  strife  for  place  in  the 
crowd  of  animals  makes  it  necessary  for  each  one  to  adjust 
itself 'to  the  place  it  holds.     As  the  individual  becomes 
fitted  to  its  condition,  so  must  the  species  as  a  whole.     The 
species  is  therefore  made  up  of  individuals  that  are  fitted 
or  may  become  fitted  for  the  conditions  of  life.     As  the 
stress  of  existence  becomes  more  severe,  the  individuals  fit 
to  continue  the   species   are   chosen  more   closely.     This 
choice  is  the  automatic  work  of  the  conditions  of  life,  but 
it  is  none  the  less  effective  in  its  operations,  and  in  the 
course  of  centuries  it  becomes  unerring.     When  conditions 
change,  the  perfection  of  adaptation  in  a  species  may  be 
the  cause  of  its  extinction.     If  the  need  of  a  special  fitness 
can  not  be  met   immediately,  the  species  will   disappear. 
For  example,  the  native  sheep  of  England  have  developed 
a  long  wool  fitted  to  protect  them  in  a  cool,  damp  climate. 
Such  sheep  transferred  to  Cuba  died  in  a  short  time,  leav- 
ing no  descendants.     The  warm  fleece,  so  useful  in  Eng- 
land, rendered  them  wholly  unfit  for  survival  in  the  tropics. 
It  is  one  advantage  of  man,  as  compared  with  other  forms 
of  life,  that  so  many  of  his  adaptations  are  external  to  his 
structure,  and  can  be  cast  aside  when  necessity  arises. 

75.  Classification  of  adaptations.— The  various  forms  of 
adaptations  may  be  roughly  divided  into  five  classes,  as  fol- 
lows :  (a)  food  securing,  (#)  self-protection,  (c)  rivalry,  (d) 
defense  of  young,  (e)  surroundings. 

The   few  examples  which  are  given  under  each  class, 

123 


124  ANIMAL  LIFE 

some  of  them  striking,  some  not  especially  so,  are  mostly 
chosen  from  the  vertebrates  and  from  the  insects,  because 
these  two  groups  of  animals  are  the  groups  with  which  be- 
ginning students  of  zoology  are  likely  to  be  familiar,  and 
the  adaptations  referred  to  are  therefore  most  likely  to  be 
best  appreciated.  Quite  as  good  and  obvious  examples  could 
be  selected  from  any  other  groups  of  animals.  The  student 


FIG.  54.— The  deep-sea  angler  (C'oiynolophus  reinhardti),  which  has  a  dorsal  spine 
modified  to  be  a  luminous  "fishing  rod  and  lure,"  attracting  lantern-fishes 
(Echiostoma  and  ^Ethophora).  An  extraordinary  adaptation  for  securing  food. 
(The  angler  is  drawn  after  a  figure  of  LUTKEN'S.) 

will  find  good  practice  in  trying  to  discover  examples  shown 
by  the  animals  with  which  he  may  be  familiar.  That  all 
or  any  part  of  the  body  structure  of  any  animal  can  be 
called  with  truth  an  example  of  adaptation  is  plain  from 
what  we  know  of  how  the  various  organs  of  the  animal 
body  have  come  to  exist.  But  by  giving  special  attention 
to  such  adaptations  as  are  plainly  obvious,  beginning  stu- 


ADAPTATIONS 


125 


dents  may  be  put  in  the 
way  of  independent  ob- 
servation along  an  ex- 
tremely interesting  and 
attractive  line  of  zoolog- 
ical study. 

76.  Adaptations  for 
securing  food.  —  For  the 
purpose  of  capture  of 
their  prey,  some  carniv- 
orous animals  are  pro- 
vided with  strong  claws, 
sharp  teeth,  hooked 
beaks,  and  other  struc- 
tures familiar  to  us  in 
the  lion,  tiger,  dog,  cat, 
owl,  and  eagle.  Insect- 
eating  mammals  have 
contrivances  especially 


FIG.  55.— The  brown  pelican,  showing  gular 
sac,  which  it  uses  in  catching  and  holding 
fishes  that  form  its  food. 


FIG.  56. — Foot  of  the  bald  eagle,  show- 
ing claws  for  seizing  its  prey. 
(CH4PMAN.) 


adapted  for  the  catching  of  insects.  The 
ant-eater,  for  example,  has  a 
curious,  long  sticky  tongue 
which  it  thrusts  forth  from 
its  cylindrical  snout  deep 
into  the  recesses  of  the  ant- 
hill, bringing  it  out  with  its 
sticky  surface  covered  with 
ants.  Animals  which  feed  on 
nuts  are  fitted  with  strong 
teeth  or  beaks  for  crack- 
ing them.  Similar  teeth  are 
found  in  those  fishes  which 
feed  on  crabs,  snails,  or  sea-ur- 
chins. Those  mammals  like 
the  horse  and  cow,  that 
feed  on  plants,  have  usually 


FIG.  57.— Giraffes  feeding. 


ADAPTATIONS 


127 


broad  chisel-like  incisor  teeth  for  cutting  off  the  foliage, 
and  teeth  of  very  similar  form  are  developed  in  the  dif- 
ferent   groups   of  plant- 
eating    fishes.       Molar 
teeth  are  found  when  it 


FIG.  58.— Scorpion,  showing  the  special  devel- 
opment of  certain  mouth  parts  (the  maxil- 
lary palpi)  as  pincer-like  organs  for  grasp- 
ing prey.  At  the  posterior  tip  of  the  body 
is  the  poisonous  sting. 


FIG.  59.— Head  of  mosquito  (fe- 
male), showing  the  piercing 
needle-like  mouth  parts  which 
compose  the  "bill." 


is  necessary  that  the  food  should  be  crushed  or  chewed, 

and  the   sharp   canine  teeth  go   with  a  flesh  diet.     The 

long  neck   of    the    giraffe 

(Fig.   57)   enables   it   to 

browse   on    the    foliage   of 

trees. 

Insects  like  the  leaf- 
beetles  and  the  grasshop- 
pers, that  feed  on  the 
foliage  of  plants,  have  a 

.          .       .  FIG.  60.— The  praying-horse  (Mantis)  with 

pair    Of      jaWS,     broad      but         fore  legs  developed  as  grasping  organs. 


128 


ANIMAL  LIFE 


sharply  edged,  for  cutting  off  bits  of  leaves  and  stems. 
Those  which  take  only  liquid  food,  as  the  butterflies  and 
sucking-bugs,  have  their  mouth  parts  modified  to  form  a 
slender,  hollow  sucking  beak  or  proboscis,  which  can  be 

thrust  into  a  flower  nectary, 
or  into  the  green  tissue  of 
plants  or  the  flesh  of  animals, 
to  suck  up  nectar  or  plant  sap 
or  blood,  depending  on  the 
special  food  habits  of  the  in- 
sect. The  honey-bee  has  a 
very  complicated  equipment 
of  mouth  parts  fitted  for  tak- 
ing either  solid  food  like  pol- 
len, or  liquid  food  like  the 
nectar  of  flowers.  The  mos- 
quito has  a  "bill"  (Fig.  59) 
composed  of  six  sharp,  slender 
needles  for  piercing  and  lac- 
erating the  flesh,  and  a  long 
tubular  under  lip  through 
which  the  blood  can  flow  into 
the  mouth.  Some  predaceous 
insects,  as  the  praying-horse 
(Fig.  60),  have  their  fore  legs 
developed  into  formidable 
grasping  organs  for  seizing  and 
holding  their  prey. 

77.  Adaptation    for     self-de- 
fense.— For  self -protection,  car- 

FIG.  61.— Acorns  put  into  bark  of  tree         .  r 

by   the   caiifomian   woodpecker    nivorous  animals  use  the  same 

(Melanerpes  formicivorus  bairdii).     weapOnS   to  defend   themselves 
— From  photograph,  Stanford  Uni-  .  . 

versity,  California.  which     Serve     to     SCCUre     their 

prey;    but    these    as    well   as 

other  animals  may  protect  themselves  in  other  fashions. 
Most  of  the  hoofed  animals  are  provided  with  horns,  struc- 


ADAPTATIONS 


129 


FIG.  62.— Section  of  bark  of  live  oak  tree  with  acorns  placed  in  it  by  the  Californian 
woodpecker  (Melanerpes  formicivorus  bairdii).— From  photograph,  Stanford 
University,  California. 

tures  useless  in  procuring  food  but  often  of  great  effective- 
ness as  weapons  of  defense.  To  the  category  of  structures 
useful  for  self-defense  belong  the  many  peculiarities  of  col- 
oration known  as  "recognition  marks."  These  are  marks, 
10 


130 


ANIMAL  LIFE 


not  otherwise  useful,  which,  are  supposed  to  enable  mem- 
bers of  any  one  species  to  recognize  their  own  kind  among 
the  mass  of  animal  life.  To  this  category  belongs  the 
black  tip  of  the  weasel's  tail,  which  re- 
mains the  same  whatever  the  changes 
in  the  outer  fur.  Another  example  is 
seen  in  the  white  outer  feathers  of  the 
tail  of  the  meadow-lark  as  well  as  in 
certain  sparrows  and  warblers.  The 
white  on  the  skunk's  back  and  tail 
serves  the  same  purpose  and  also  as  a 
warning.  It  is  to  the  skunk's  advan- 
tage not  to  be  hidden,  for  to  be  seen  in 
the  crowd  of  animals  is  to  be  avoided 
by  them.  The  songs  of  birds  and  the 
calls  of  -various  creatures  serve  also  as 
recognition  marks.  Each  species  knows 
and  heeds  its  own  characteristic  song 
or  cry,  and  it  is  a  source  of  mutual 
protection.  The  fur-seal  pup  knows 
its  mother's  call,  even  though  ten  thou- 
sand other  mothers  are  calling  on  the 
rookery. 

The  ways  in  which  animals  make 
themselves  disagreeable  or  dangerous 
to  their  captors  are  almost  as  varied  as  the  animals  them- 
selves. Besides  the  teeth,  claws,  and  horns  of  ordinary 
attack  and  defense,  we  find  among  the  mammals  many 
special  structures  or  contrivances  which  serve  for  de- 
fense through  making  their  possession  unpleasant.  The 
scent  glands  of  the  skunk  and  its  relatives  are  noticed 
above.  The  porcupine  has  the  bristles  in  its  fur  specialized 
as  quills,  barbed  and  detachable.  These  quills  fill  the 
mouth  of  an  attacking  fox  or  wolf,  and  serve  well  the  pur- 
pose of  defense.  The  hedgehog  of  Europe,  an  animal  of 
different  nature,  being  related  rather  to  the  mole  than  to 


FIG.  63.— Centiped.  The 
foremost  pair  of  legs  is 
modified  to  be  a  pair  of 
seizing  and  stinging  or- 
gans. An  adaptation 
for  self-defense  and  for 
securing  food. 


ADAPTATIONS 


131 


the  squirrel,  has  a  similar  armature  of  quills.     The  armadillo 
of  the  tropics  has  movable  shields,  and  when  it  withdraws  its 


FIG.  64. — Flying  fishes.  (The  upper  one  a  species  of  Cyp&durus,  the  lower  of  Exocce,- 
tus.)  These  fishes  escape  from  their  enemies  by  leaping  into  the  air  and  sailing 
or  "flying"  long  distances. 

head  (which  is  also  defended  by  a  bony  shield)  it  is  as  well 
protected  as  a  turtle. 


FIG.  65.— The  horned  toad  (Phi~ynosoma  blainvillei).    The  spiny  covering  repels  many 

enemies. 

Special  organs  for  defense  of  this  nature  are  rare  among 
birds,  but  numerous  among  reptiles.     The  turtles  are  all 


132 


ANIMAL  LIFE 


protected  by  bony  shields,  and  some  of  them,  the  box-tur- 
tles, may  close  their  shields  almost  hermetically.  The 
snakes  broaden  their  heads,  swell  their  necks,  or  show  their 
forked  tongues  to  frighten  their  enemies.  Some  of  them 


FIG.  66.— Nokee  or  poisonous  scorpion-fish  (Emmydrichthys  vulcanus)  with  poison- 
ous spines,  from  Tahiti. 

are  further  armed  with  fangs  connected  with  a  venom  gland, 
so  that  to  most  animals  their  bite  is  deadly.  Besides  its 
fangs  the  rattlesnake  has  a  rattle  on  the  tail  made  up  of  a 


FIG.  67. — Mad  torn  (Schilbeodes  furiosus)  with  poisoned  pectoral  spine. 

succession  of  bony  clappers,  modified  vertebrae,  and  scales, 
by  which  intruders  are  warned  of  their  presence.  This 
sharp  and  insistent  buzz  is  a  warning  to  animals  of  other 
species  and  a  recognition  signal  to  those  of  its  own  kind. 


ADAPTATIONS  133 

Even  the  fishes  have  many  modes  of  self-defense  through 
giving  pain  or  injury  to  those  who  would  swallow  them. 
The  cat-fishes  or  horned  pouts  when  attacked  set  immov- 
ably the  sharp  spine  of  the 
pectoral  fin,  inflicting  a 
jagged  wound.  Pelicans 
who  have  swallowed  a  cat- 
fish have  heen  known  to 
die  of  the  wounds  inflicted 
hy  the  fish's  spine.  In 
the  group  of  scorpion- 
fishes  and  toad-fishes  are 
certain  genera  in  which 
these  spines  are  provided 
with  poison  glands.  These 
may  inflict  very  severe 
wounds  to  other  fishes,  or 
even  to  birds  or  man.  One  of  this  group 
of  poison-fishes  is  the  nokee  (Emmydrich- 
thys,  Fig.  66).  A  group  of  small  fresh- 
water cat-fishes,  known  as  the  mad  toms 
(Fig.  67),  have  also  a  poison  gland  attached 
to  the  pectoral  spine,  and  its  sting  is  most 
exasperating,  like  the  sting  of  a  wasp. 
The  sting-rays  (Fig.  68)  of  many  species  FIG.  ea— A  sting-ray 
have  a  strong,  jagged  spine  on  the  tail,  JEXLT** 
covered  with  slime,  and  armed  with  broad 
saw -like  teeth.  This  inflicts  a  dangerous  wound,  not 
through  the  presence  of  specific  venom,  but  from  the  dan- 
ger of  blood  poisoning  arising  from  the  slime,  and  the 
ragged  or  unclean  cut. 

Many  fishes  are  defended  by  a  coat  of  mail  or  a  coat  of 
sharp  thorns.  The  globe-fishes  and  porcupine-fishes  (Fig. 
69)  are  for  the  most  part  defended  by  spines,  but  their 
instinct  to  swallow  air  gives  them  an  additional  safeguard. 
"When  ojie  of  these  fishes  is  disturbed  it  rises  to  the  surface, 


134 


ANIMAL  LIFE 


gulps  air  until  its  capacious  stomach  is  filled,  and  then 
floats  belly  upward  on  the  surface.  It  is  thus  protected 
from  other  fishes,  though  easily  taken  by  man.  The  torpe- 
do, electric  eel,  electric  cat-fish,  and  star-gazer,  surprise  and 

stagger  their  captors  by 
means  of  electric  shocks. 
In  the  torpedo  or  electric 
ray  (Fig.  70),  found  on 
the  sandy  shores  of  all 
warm  seas,  on  either  side 
of  the  head  is  a  large 
honeycomb-like  structure 
which  yields  a  strong 
electric  shock  whenever 
the  live  fish  is  touched. 
This  shock  is  felt  severe- 
ly if  the  fish  be  stabbed 
with  a  knife  or  metallic 
spear.  The  electric  eel 
of  the  rivers  of  Para- 
guay and  southern  Bra- 
zil is  said  to  give  severe 
shocks  to  herds  of  wild 
horses  driven  through 
the  streams,  and  similar 
accounts  are  given  of  the 
electric  cat-fish  of  the 
Nile. 

Among    the    insects, 

one  floating   belly  upward,  with   inflated     the    possession    of    stingS 

SS^aWD  fr°m  8PeCimen8  ^  ^    is  not  uncommon.      The 

wasps  and  bees  are  fa- 
miliar examples  of  stinging  insects,  but  many  other  kinds, 
less  familiar,  are  similarly  protected.  All  insects  have 
their  bodies  covered  with  a  coat  of  armor,  composed  of  a 
horny  substance  called  chitin.  In  some  cases  this  chitin- 


FIG.  69. — Porcupine-fish  (Diodon  hystrix),  the 
lower  ones  swimming  normally,  the  upper 


ADAPTATIONS 


135 


ous  coat  is  very  thick  and  serves  to  protect  them  effectu- 
ally. This  is  especially  true  of  the  beetles.  Some  insects 
are  inedible  (as  mentioned  in  Chapter  XII),  and  are  con- 
spicuously colored  so  as  to  be  readily  recognized  by  in- 
sectivorous birds.  The  birds,  knowing  by  experience  that 
these  insects  are  ill-tasting,  avoid  them.  Others  are  ef- 
fectively concealed  from  their  enemies  by  their  close 
resemblance  in  color  and  marking  to  their  surroundings. 
These  protective  resem- 
blances are  discussed  in 
Chapter  XII. 

78.  Adaptation  for  rivalry. 
— In  questions  of  attack  and 
defense,  the  need  of  meeting 
animals  of  their  own  kind  as 
well  as  animals  of  other 
races  must  be  considered.  In 
struggles  of  species  with 
those  of  their  own  kind,  the 
term  rivalry  may  be  applied. 
Actual  warfare  is  confined 
mainly  to  males  in  the  breed- 
ing season  and  to  polyga- 
mous animals.  Among  those 
in  which  the  male  mates 
with  many  females,  he  must 
struggle  with  other  males  for 
their  possession.  In  all  the 
groups  of  vertebrates  the 
sexes  are  about  equal  in  num- 
bers. Where  mating  exists, 
either  for  the  season  or  for 

life,  this  condition  does  not   involve   serious   struggle   or 
destructive  rivalry. 

Among  monogamous  birds,  or  those  which   pair,  the 
male  courts  the  female  of  his  choice  by  song  and  by  display 


FIG.  70.— Torpedo  or  electric  ray  (Nar- 
cine  brasiliensis),  showing  electric 
cells. 


ADAPTATIONS 


137 


of  his  bright  feathers.  The  female  consents  to  be  chosen 
by  the  one  which  pleases  her.  It  is  believed  that  the  hand- 
somest, most  vivacious,  and  most  musical  males  are  the 
ones  most  successful  in  such  courtship.  With  polygamous 
animals  there  is  intense  rivalry  among  the  males  in  the 
mating  season,  which  in  almost  all  species  is  in  the  spring. 
The  strongest  males  survive  and  reproduce  their  strength. 
The  most  notable  adaptation  is  seen  in  the  superior  size 
of  teeth,  horns,  mane,  or  spurs.  Among  the  polygamous 
fur  seals  (Fig.  71)  and  sea  lions  the  male  is  about  four  times 


FIG.  72.— A  wild  duck  (Ay  thy  a)  family.    Male,  female,  and  prsecocial  young. 

the  size  of  the  female.  In  the  polygamous  family  of  deer, 
buffalo,  and  the  domestic  cattle  and  sheep,  the  male  is  larger 
and  more  powerfully  armed  than  the  female.  In  the  polyg- 
amous group  to  which  the  hen,  turkey,  and  peacock  belong 
the  males  possess  the  display  of  plumage,  and  the  structures 
adapted  for  fighting,  with  the  will  to  use  them. 

79.  Adaptations  for  the  defense  of  the  young. — The  pro- 
tection of  the  young  is  the  source  of  many  adaptive  struc- 
tures as  well  as  of  the  instincts  by  which  such  structures  are 


138 


ANIMAL  LIFE 


utilized.     In  general,  those  animals  are  highest  in  develop- 
ment, with  best  means  of  holding  their  own  in  the  struggle 


FIG.  73.— The  altricial  nestlings  of  the  Blue  jay  (Cyanocitta 

for  life,  that  take  best  care  of  their  young.     The  homes 
of  animals  are  elsewhere  specially  discussed  (see  Chapter 


ADAPTATIONS 


139 


XV),  but  those  instincts  which  lead  to  home-building 
may  all  be  regarded  as  useful  adaptations  in  preserving  the 
young.  Among  the  lower  or  more  coarsely  organized 


IG.  74.— Kangaroo  (Macropus  rufus)  with  youug  iu  pouch. 


140 


ANIMAL  LIFE 


birds,  such  as  the  chicken,  the  duck,  and  the  auk,  as  with 
the  reptiles,  the  young  animal  is  hatched  with  well-devel- 
oped muscular  system  and  sense 
organs,  and  is  capable  of  running 
about,  and,  to  some  extent,  of  feed- 
ing itself.  Birds  of  this  type  are 
known  as  prcecocial  (Fig.  72),  while 
the  name  altricial  (Fig.  73)  is  ap- 
plied to  the  more  highly  organized 
forms,  such  as  the  thrushes,  doves, 
and  song-birds  generally.  With 
these  the  young  are  hatched  in  a 
wholly  helpless  condition,  with  in- 
effective muscles,  deficient  senses, 
and  dependent  wholly  upon  the 
parent.  The  altricial  condition  de- 
mands the  building  of  a  nest,  the 
establishment  of  a  home,  and  the 
continued  care  of  one  or  both  of 

lata)  cut  open  to  show  young    the  parents. 

^atoSn"       TheverylowestmammaUknown, 
the    duck-bills    (Monotremes)    of 

Australia,  lay  large  eggs  in  a  strong  shell  like  those  of  a 
turtle,  and  guard  them  with  great  jealousy.  But  with 
almost  all  mammals  the  egg  is  very  small  and  without 
much  food-yolk.  The  egg  begins  its  development  within 
the  body.  It  is  nourished  by  the 
blood  of  the  mother,  and  after  birth 
the  young  is  cherished  by  her,  and 
fed  by  milk  secreted  by  specialized 
glands  of  the  skin.  All  these  features 
are  adaptations  tending  toward  the 
preservation  of  the  young.  In  the 

division  of  mammals  next  lowest  to  the  Monotremes — the 
kangaroo,  opossum,  etc. — the  young  are  born  in  a  very  im- 
mature state  and  are  at  once  seized  by  the  mother  and 


ill! 


FIG.  76.— Egg-case  of  the  cock- 
roach. 


ADAPTATIONS 


141 


thrust  into  a  pouch  or  fold  of  skin  along  the  abdomen, 
where  they  are  kept  until  they  are  able  to  take  care  of 
themselves  (Fig.  74).  This  is  an  interesting  and  ingenious 
adaptation,  but  less  specialized  and 
less  perfect  an  adaptation  than  the 
conditions  found  in  ordinary  mam- 
mals. 

Among  the  insects,  the  special 
provisions  for  the  protection  and 
care  of  the  eggs  and  the  young  are 
wide-spread  and  various.  Some  of 
those  adaptations  which  take  the 
special  form  of  nests  or  "homes" 
will  be  described  in  a  later  chapter 

(see    Chapter    XV).        The    eggS    of    FIG-  77.— Giant  water-bug  (Ser- 

the  common  cockroach  are  laid  in  ^£  J£le  carrying  egg8 
small  packets  inclosed  in  a  firm  wall 

(Fig.  76).  The  eggs  of  the  great  water-bugs  are  carried  on 
the  back  of  the  male  (Fig.  77) ;  and  the  spiders  lay  their 
eggs  in  a  silken  sac  or  cocoon,  and  some  of  the  ground  or 


FIG.  78.— Cocoon  inclosing  the  pupa  of  the  great  Ceanothus  moth.    Spun  of  silk  by  the 
larva  before  pupation. 

running  spiders  (Lycosidcp)  drag  this  egg-sac,  attached  to 
the  tip  of  the  abdomen,  about  with  them.  The  young 
spiders  when  hatched  live  for  some  days  inside  this  sac, 
feeding  on  each  other!  Many  insects  have  long,  sharp, 


142 


ANIMAL  LIFE 


piercing  ovipositors,  by  means  of  which  the  eggs  are  de- 
posited in  the  ground  or  in  the  leaves  or  stems  of  green 
plants,  or  even  in  the  hard  wood  of  tree-trunks.  Some  of 


the  scale  insects  se- 
crete wax  from  their 
bodies  and  form  a 
large,  often  beautiful  • 
egg-case,  attached  to 
and  nearly  covering  the  body  in 
which  eggs  are  deposited  (Fig. 
79).  The  various  gall  insects  lay 
their  eggs  in  the  soft  tissue  of 
plants,  and  on  the  hatching  of 
the  larvae  an  abnormal  growth 
of  the  plant  occurs  about  the 
young  insect,  forming  an  in- 
closing gall  that  serves  not  only 
to  protect  the  insect  within, 
but  to  furnish  it  with  an  abun- 
dance of  plant-sap,  its  food.  The 
young  insect  remains  in  the  gall 
until  it  completes  its  develop- 
ment and  growth,  when  it 
gnaws  its  way  out.  Such  insect  galls  are  especially  abun- 
dant on  oak  trees  (Fig.  80).  The  care  of  the  eggs  and  the 
young  of  the  social  insects,  as  the  bees  and  ants,  are  de- 
scribed in  Chapter  IX. 


FIG.  79. — The  cottony  cushion  scale 
insect  (Icerya  purchasi),  from 
California.  The  male  is  winged, 
the  female  wingless  and  with  a 
large  waxen  egg-sac  (e.s.)  attached 
to  her  body.  (The  lines  at  the  left 
of  each  figure  indicate  the  size  of 
the  insects.) 


ADAPTATIONS  143 

80.  Adaptations  concerned  with  surroundings  in  life. — A 

large  part  of  the  life  of  the  animal  is  a  struggle  with  the 
environment  itself;  in  this  struggle  only  those  that  are 
adapted  live  and  leave  descendants  fitted  like  themselves. 
The  fur  of  mammals  fits  them  to  their  surroundings.  As 
the  fur  differs,  so  may  the  habits  change.  Some  animals 
are  active  in  winter ;  others,  as  the  bear,  hibernate,  sleep- 
ing in  caves  or  hollow  trees  or  in  burrows  until  conditions 
are  favorable  for  their  activity.  Most  snakes  and  lizards 
hibernate  in  cold  weather.  In  the  swamps  of  Louisiana, 


FIG.  80.— The  giant  gall  of  the  white  oak  (California),  made  by  the  gall  insect  Andri- 
cus  californicus.  The  gall  at  the  right  cut  open  to  show  tunnels  made  by  the 
insects  in  escaping  from  the  gall. — From  photograph. 

in  winter,  the  bottom  may  often  be  seen  covered  with  water 
snakes  lying  as  inert  as  dead  twigs.  Usually,  however, 
hibernation  is  accompanied  by  concealment.  Some  animals 
in  hibernation  may  be  frozen  alive  without  apparent  injury. 
The  blackfish  of  the  Alaska  swamps,  fed  to  dogs  when 
frozen  solid,  has  been  known  to  revive  in  the  heat  of  the 
dog's  stomach  and  to  wriggle  out  and  escape.  As  animals 
resist  heat  and  cold  by  adaptations  of  structure  or  habits, 
so  may  they  resist  dryness.  Certain  fishes  hold  reservoirs 


144  ANIMAL  LIFE 

of  water  above  their  gills,  by  means  of  which  they  can 
breathe  during  short  excursions  from  the  water.  Still 
others  (mud-fishes)  retain  the  primitive  lung-like  structure 
of  the  swim-bladder,  and  are  able  to  breathe  air  when,  in  the 
dry  season,  the  water  of  the  pools  is  reduced  to  mud. 

Another  series  of  adaptations  is  concerned  with  the 
places  chosen  by  animals  for  their  homes.  The  fishes  that 
live  in  water  have  special  organs  for 
breathing  under  water  (Fig.  82). 
Many  of  the  South  American  mon- 
keys have  the  tip  of  the  tail  adapted 
for  clinging  to  limbs  of  trees  or  to 
the  bodies  of  other  monkeys  of  its 
own  kind.  The  hooked  claws  of  the 
bat  hold  on  to  rocks,  the  bricks  of 
chimneys,  or  to  the  surface  of  hollow 
trees  where  the  bat  sleeps  through 
the  day.  The  tree-frogs  (Fig.  83)  or 
tree-toads  have  the  tips  of  the  toes 
swollen,  forming  little  pads  by  which 
they  cling  to  the  bark  of  trees. 

Among  other  adaptations  relat- 
ing to  special  surroundings  or  con- 
ditions of  life  are  the  great  cheek 
pouches  of  the  pocket  gophers, 
which  carry  off  the  soil  dug  up  by 
the  large  shovel-like  feet  when  the 
gopher  excavates  its  burrow. 

Those  insects  which  live  under- 

FIQ.  81.— Insect  galls  on  leaf. 

ground,  making  burrows  or  tunnels 

in  the  soil,  have  their  legs  or  other  parts  adapted  for  dig- 
ging and  burrowing.  The  mole  cricket  (Fig.  84)  has  its 
legs  stout  and  short,  with  broad,  shovel-like  feet.  Some 
water-beetles  (Fig.  85)  and  water-bugs  have  one  or  more  of 
the  pairs  of  legs  flattened  and  broad  to  serve  as  oars  or  pad. 
dies  for  swimming.  The  grasshoppers  or  locusts,  who  leap, 


ADAPTATIONS 


145 


have  their  hind  legs  greatly  enlarged  and  elon- 
gated, and  provided  with  strong  muscles,  so  as 
to  make  of  them  "leaping  legs."     The  grubs 


FIG.  82. — Head  of  rainbow  tront  (Salmo  irideus)  with  gill  cover  bent  back  to  show 
gills,  the  breathing  organs. 

or  larvae  of  beetles  which  live  as  "  borers "  in  tree-trunks 
have  mere  rudiments  of  legs,  or  none  at  all  (Fig.  86). 
They  have  great,  strong,  biting  jaws  for  cutting  away 
the  hard  wood.  They  move  simply  by  wriggling  along 
in  their  burrows  or  tunnels. 

Insects    that    live 
in  water  either  come 
up  to  the  surface  to 
breathe  or  take  down 
air   underneath   their 
wings,     or     in     some 
other    way,    or    have 
gills  for  breathing  the 
air    which    is    mixed 
with  the  water.    These 
gills  are  special  adap- 
tive structures  which  present  a  great  variety  of  form  and 
appearance.     In  the  young  of  the  May-flies  they  are  deli- 
cate plate-like  flaps  projecting  from  the  sides  of  the  body. 
They  are  kept  in  constant  motion,  gently  waving  back  and 
11 


FIG.  83.— Tree-toad  (Hyla  regilla). 


146 


ANIMAL  LIFE 


forth  in  the  water  so  as  to  maintain  currents  to  bring  fresh 
water  in  contact  with  them.     Young  mosquitoes  (Fig.  87) 

do  not  have  gills,  but  come 
up  to  the  surface  to  breathe. 
The  larvae,  or  wrigglers, 
breathe  through  a  special 


FIG.  84.— The  mole  cricket  (Gryllotalpa), 
with  fore  feet  modified  for  digging. 


FIG.  85.— A  water-beetle  (Hydroph- 
Uus). 


tube  at  the  posterior  tip  of  the  body,  while  the  pupae  have 
a  pair  of  horn-like  tubes  on  the  back  of  the  head  end  of 
the  body. 

81.  Degree  of  structural  change  in  adaptations. — While 
among  the  higher  or  vertebrate  animals,  especially  the 
fishes  and  reptiles,  most  remarkable  cases  of  adaptations 
occur,  yet  the  structural  changes  are  for  the  most  part  ex- 
ternal, never  seriously  affecting  the  development  of  the 
internal  organs  other 
than  the  skeleton.  The 
organization  of  these 
higher  animals  is  much 
less  plastic  than  among 
the  invertebrates.  In 
general,  the  higher  the  type  the  more  persistent  and  un- 
changeable are  those  structures  not  immediately  exposed 


FIG.  86.— Wood-boring  beetle  larva  (Prionus). 


ADAPTATIONS 


147 


to  the  influence  of  the  struggle  for  existence.  It  is  thus 
the  outside  of  an  animal  that  tells  where  its  ancestors 
have  lived.  The  inside,  suffering  little  change,  whatever 
the  surroundings,  tells  the  real  nature  of  the  animal. 

82.  Vestigial  organs. — In  general,  all  the  peculiarities  of 
animal  structure  find  their  explanation  in  some  need  of 
adaptation.  When  this  need  ceases,  the  structure  itself 
tends  to  disappear  or  else  to  serve  some  other  need.  In 
the  bodies  of  most  animals  there  are  certain  incomplete 
or  rudimentary  organs 
or  structures  which 
serve  no  distinct  use- 
ful purpose.  They  are 
structures  which,  in  the 
ancestors  of  the  ani- 
mals now  possessing 
them,  were  fully  devel- 
oped functional  organs, 
but  which,  because  of  a 
change  in  habits  or  con- 
ditions of  living,  are  of 
no  further  need,  and 
are  gradually  dying  out. 
Such  organs  are  called 
vestigial  organs.  Ex- 
amples are  the  disused 
ear  muscles  of  man,  the 
vermiform  appendix  in 
man,  which  is  the  reduced  and  now  useless  anterior  end 
of  the  large  intestine.  In  the  lower  animals,  the  thumb  or 
degenerate  first  finger  of  the  bird  with  its  two  or  three  little 
quills  serves  as  an  example.  So  also  the  reduced  and  elevated 
hind  toe  of  certain  birds,  the  splint  bones  or  rudimentary 
side  toes  of  the  horse,  the  rudimentary  eyes  of  blind  fishes, 
the  minute  barbel  or  beard  of  the  horned  dace  or  chub,  and 
the  rudimentary  teeth  of  the  right  whales  and  sword-fish. 


FIG.    87. — Young   stages   of   the   mosquito. 
a,  larva  (wriggler) ;  b,  pupa. 


148 


ANIMAL  LIFE 


Each  of  these  vestigial  organs  tells  a  story  of  some  past 
adaptation  to  conditions,  one  that  is  no  longer  needed  in 
the  life  of  the  species.  They  have  the  same  place  in  the 
study  of  animals  that  silent  letters  have  in  the  study  of 
words.  For  example,  in  our  word  knight  the  Ic  and  gh  are 
no  longer  sounded ;  but  our  ancestors  used  them  both,  as 
the  Germans  do  to-day  in  their  cognate  word  Kneclit.  So 
with  the  French  word  temps,  which  means  time,  in  which 
both  p  and  s  are  silent.  The  Eomans,  from  whom  the 
French  took  this  word,  needed  all  its  letters,  for  they  spelled 
and  pronounced  it  tempus.  In  general,  every  silent  letter 
in  every  word  was  once  sounded.  In  like  manner,  every 
vestigial  structure  was  once  in  use  and  helpful  or  necessary 
to  the  life  of  the  animal  which  possessed  it. 


:1 


Horns  of  two  male  deer  interlocked  while  fighting.    Permission  of  G.  O.  SHIELDSS 
publisher  of  Recreation. 


CHAPTER  IX 

ANIMAL   COMMUNITIES   AND   SOCIAL   LIFE 

83.  Man  not  the  only  social  animal — Man  is  commonly 
called  the  social  animal,  but  he  is  not  the  only  one  to 
which  this  term  may  be  applied.     There  are  many  others 
which  possess  a  social  or  communal  life.     A  moment's 
thought  brings  to  mind  the  familiar  facts  of  the  communal 
life  of  the  honey-bee  and  of  the  ants.    And  there  are  many 
other  kinds  of  animals,  not  so  well  known  to  us,  that  live 
in  communities  or  colonies,  and  live  a  life  which  in  greater 
or  less  degree  is  communal  or  social.     In  this  connection 
we  may  use  the  term  communal  for  the  life  of  those  ani- 
mals in  which  the  division  of  labor  is  such  that  the  indi- 
vidual is  dependent  for  its  continual  existence  on  the  com- 
munity as  a  whole.     The  term  social  life  would  refer  to  a 
lower  degree  of  mutual  aid  and  mutual  dependence. 

84.  The  honey-bee. — Honey-bees    live    together,  as  we 
know,  in  large  communities.     We  are  accustomed  to  think 
of  honey-bees  as  the  inhabitants  of  bee-hives,  but  there 
were  bees  before   there  were  hives.     The  "bee-tree"  is 
familiar  to  many  of  us.     The  bees,  in  Nature,  make  their 
home  in  the  hollow  of  some  dead  or  decaying  tree-trunk, 
and  carry  on  there  all  the  industries  which  characterize 
the  busy  communities  in  the   hives.     A  honey-bee   com- 
munity comprises  three  kinds  of  individuals  (Fig.  88) — 
namely,   a  fertile   female    or   queen,   numerous   males   or 
drones,   and  many  infertile   females   or  workers.     These 
three  kinds  of  individuals  differ  in  external  appearance 
sufficiently  to  be  readily  recognizable.     The  workers  are 

149 


ANIMAL  LIFE 


smaller  than  the  queens  and  drones,  and  the  last  two  differ 
in  the  shape  of  the  abdomen,  or  hind  body,  the  abdomen  of 
the  queen  being  longer  and  more  slender  than  that  of  the 


FIG.  88.— Honey-bee,    a,  drone  or  male  ;  b,  worker  or  infertile  female  ;  c,  queen  or 
fertile  female. 

male  or  drone.  In  a  single  community  there  is  one  queen, 
a  few  hundred  drones,  and  ten  to  thirty  thousand  workers. 
The  number  of  drones  and  workers  varies  at  different 
times  of  the  year,  being  smallest  in  winter.  Each  kind  of 
individual  has  certain  work  or  business  to  do  for  the  whole 
community.  The  queen  lays  all  the  eggs  from  which  new 
bees  are  born;  that  is,  she  is  the  mother  of  the  entire 
community.  The  drones  or  males  have  simply  to  act  as 
royal  consorts ;  upon  them  depends  the  fertilization  of  the 
eggs.  The  workers  undertake  all  the  food-getting,  the 
care  of  the  young  bees,  the  comb-building,  the  honey-mak- 
ing— all  the  industries  with  which  we  are  more  or  less 
familiar  that  are  carried  on  in  the  hive.  And  all  the 
work  done  by  the  workers  is  strictly  work  for  the  whole 
community ;  in  no  case  does  the  worker  bee  work  for  itself 
alone ;  it  works  for  itself  only  in  so  far  as  it  is  a  member 
of  the  community. 

How  varied  and  elaborately  perfected  these  industries 
are  may  be  perceived  from  a  brief  account  of  the  life  his- 
tory of  a  bee  community.  The  interior  of  the  hollow  in 
the  bee-tree  or  of  the  hive  is  filled  with  "  comb  " — that  is, 
with  wax  molded  into  hexagonal  cells  and  supports  for 
these  cells.  The  molding  of  these  thousands  of  symmet-' 


ANIMAL  COMMUNITIES  AND  SOCIAL  LIFE         151 


rical  cells  is  accomplished  by  the  workers  by  means  of  their 
specially  modified  trowel-like  mandibles  or  jaws.  The  wax 
itself,  of  which  the  cells  are  made,  comes  from  the  bodies 
of  the  workers  in  the  form  of  small 
liquid  drops  which  exude  from  the  skin 
on  the  under  side  of  the  abdomen  or 
hinder  body  rings.  These  droplets 
run  together,  harden  and  become  flat- 
tened, and  are  removed  from  the  wax 
plates,  as  the  peculiarly  modified  parts 
of  the  skin  which  produce  the  wax 
are  called,  by  means  of  the  hind  legs, 
which  are  furnished  with  scissor-like 
contrivances  for  cutting  off  the  wax 
(Fig.  89).  In  certain  of  the  cells  are 
stored  the  pollen  and  honey,  which 
serve  as  food  for  the  community.  The 
pollen  is  gathered  by  the  workers  from 
certain  favorite  flowers  and  is  carried 
by  them  from  the  flowers  to  the  hive 
in  the  "pollen  baskets,"  the  slightly 
concave  outer  surfaces  of  one  of  the 
segments  of  the  broadened  and  flattened 
hind  legs.  This  concave  surface  is  lined 
on  each  margin  with  a  row  of  incurved 

,./»-,.  -U  •    i_    i_    i  j     j_i  -n  FIG.  89. — Posterior  leg  of 

Stin   nairS  WniCn    hold,    the    pollen    maSS        worker  honey-bee.  The 

securely  in  place  (Fig.  89).  The  "  honey  " 

is  the  nectar  of  flowers  which  has  been 

sucked  up  by  the  workers  by  means  of 

their    elaborate   lapping   and     sucking 

mouth  parts  and  swallowed  into  a  sort 

of  honey-sac  or  stomach,  then  brought 

to  the  hive  and  regurgitated  into  the 

cells.      This    nectar  is  at  first  too  watery  to    be    good 

honey,  so  the  bees  have  to  evaporate  some  of  this  water. 

Many  of  the  workers  gather  above  the  cells  containing 


concave  surface  of  the 
upper  large  joint  with 
the  marginal  hairs  is 
the  pollen  basket ;  the 
wax  shears  are  the  cut- 
ting surfaces  of  the 
angle  between  the  two 
large  segments  of  the 
leg. 


152  ANIMAL  LIFE 

nectar,  and  buzz — that  is,  vibrate  their  wings  violently. 
This  creates  currents  of  air  which  pass  over  the  exposed 
nectar  and  increase  the  evaporation  of  the  water.  The 
violent  buzzing  raises  the  temperature  of  the  bees'  bodies, 
and  this  warmth  given  off  to  the  air  also  helps  make  evap- 
oration more  rapid.  In  addition  to  bringing  in  food  the 
workers  also  bring  in,  when  necessary,  "  propolis,"  or  the 
resinous  gum  of  certain  trees,  which  they  use  in  repairing 
the  hive,  as  closing  up  cracks  and  crevices  in  it. 

In  many  of  the  cells  there  will  be  found,  not  pollen  or 
honey,  but  the  eggs  or  the  young  bees  in  larval  or  pupal 

condition  (Fig.  90). 
The  queen  moves 
about  through  the 
hive,  laying  eggs. 
She  deposits  only  one 
egg  in  a  cell.  In 
three  days  the  egg 
hatches,  and  the 
young  bee  appears 
as  a  helpless,  soft, 
white, -footless  grub 

FIG.  90.— Cells  containing  eggs,  larvse,  and  pupa;  of  or  larva.  It  is  Cared 
the  honey-bee.  The  lower  large,  irregular  cells  .  -,  pprtain  nf  ihp 
are  queen  cells.-After  BENTON.  W  C 

workers,  that  may  be 

called  nurses.  These  nurses  do  not  differ  structurally  from 
the  other  workers,  but  they  have  the  special  duty  of  caring 
for  the  helpless  young  bees.  They  do  not  go  out  for  pollen 
or  honey,  but  stay  in  the  hive.  They  are  usually  the  new 
bees — i.  e.,  the  youngest  or  most  recently  added  workers. 
After  they  act  as  nurses  for  a  week  or  so  they  take  their 
places  with  the  food-gathering  workers,  and  other  new 
bees  act  as  nurses.  The  nurses  feed  the  young  or  larval 
bees  at  first  with  a  highly  nutritious  food  called  bee-jelly, 
which  the  n arses  make  in  their  stomach,  and  regurgitate 
for  the  Iarv96.  After  the  larvas  are  two  or  three  days  old 


ANIMAL  COMMUNITIES  AND  SOCIAL  LIFE         153 

they  are  fed  with  pollen  and  honey.  Finally,  a  small  mass 
of  food  is  put  into  the  cell,  and  the  cell  is  "  capped  "  or 
covered  with  wax.  The  larva,  after  eating  all  the  food,  in 
two  or  three  days  more  changes  into  a  pupa,  which  lies 
quiescent  without  eating  for  thirteen  days,  when  it  changes 
into  a  full-grown  bee.  The  new  bee  breaks  open  the  cap 
of  the  cell  with  its  jaws,  and  comes  out  into  the  hive,  ready 
to  take  up  its  share  of  the  work  for  the  community.  In  a 
few  cases,  however,  the  life  history  is  different.  The  nurses 
will  tear  down  several  cells  around  some  single  one,  and 
enlarge  this  inner  one  into  a  great  irregular  vase-shaped 
cell.  When  the  egg  hatches,  the  grub  or  larva  is  fed  bee- 
jelly  as  long  as  it  remains  a  larva,  never  being  given  ordi- 
nary pollen  and  honey  at  all.  This  larva  finally  pupates, 
and  there  issues  from  the  pupa  not  a  worker  or  drone  bee, 
but  a  new  queen.  The  egg  from  which  the  queen  is  pro- 
duced is  the  same  as  the  other  eggs,  but  the  worker  nurses 
by  feeding  the  larva  only  the  highly  nutritious  bee-jelly 
make  it  certain  that  the  new  bee  shall  become  a  queen 
instead  of  a  worker.  It  is  also  to  be  noted  that  the  male 
bees  or  drones  are  hatched  from  eggs  that  are  not  ferti- 
lized, the  queen  having  it  in  her  power  to  lay  either  ferti- 
lized or  unfertilized  eggs.  From  the  fertilized  eggs  hatch 
larvae  which  develop  into  queens  or  workers,  depending  on 
the  manner  of  their  nourishment;  from  the  unfertilized 
eggs  hatch  the  males. 

When  several  queens  appear  there  is  much  excitement 
in  the  community.  Each  community  has  normally  a  single 
one,  so  that  when  additional  queens  appear  some  rearrange- 
ment is  necessary.  This  rearrangement  comes  about  first 
by  fighting  among  the  queens  until  only  one  of  the  new 
queens  is  left  alive.  Then  the  old  or  mother  queen  issues 
from  the  hive  or  tree  followed  by  many  of  the  workers. 
She  and  her  followers  fly  away  together,  finally  alighting 
on  some  tree  branch  and  massing  there  in  a  dense  swarm. 
This  is  the  familiar  phenomenon  of  "swarming."  The 


154:  ANIMAL  LIFE 

swarm  finally  finds  a  new  hollow  tree,  or  in  the  case  of  the 
hive-bee  (Fig.  91)  the  swarm  is  put  into  a  new  hive,  where 
the  bees  build  cells,  gather  food,  produce  young,  and  thus 


FIG.  91. — Hiving  a  swarm  of  honey-bees.    Photograph  by  S.  J.  HUNTER. 

found  a  new  community.  This  swarming  is  simply  an  emi- 
gration, which  results  in  the  wider  distribution  and  in  the 
increase  of  the  number  of  the  species.  It  is  a  peculiar  but 
effective  mode  of  distributing  and  perpetuating  the  species. 
There  are  many  other  interesting  and  suggestive  things 
which  might  be  told  of  the  life  in  a  bee  community :  how 
the  community  protects  itself  from  the  dangers  of  starva- 
tion when  food  is  scarce  or  winter  comes  on  by  killing  the 
useless  drones  and  the  immature  bees  in  egg  and  larval 
stage ;  how  the  instinct  of  home-finding  has  been  so  highly 
developed  that  the  worker  bees  go  miles  away  for  honey 
and  nectar,  flying  with  unerring  accuracy  back  to  the  hive ; 
of  the  extraordinarily  nice  structural  modifications  which 
adapt  the  bee  so  perfectly  for  its  complex  and  varied  busi- 
nesses ;  and  of  the  tireless  persistence  of  the  workers  until 


ANIMAL   COMMUNITIES  AND  SOCIAL  LIFE         155 

they  fall  exhausted  and  dying  in  the  performance  of  their 
duties.  The  community,  it  is  important  to  note,  is  a  per- 
sistent or  continuous  one.  The  workers  do  not  live  long, 
the  spring  broods  usually  not  over  two  or  three  months, 
and  the  fall  broods  not  more  than  six  or  eight  months; 
but  new  ones  are  hatching  while  the  old  ones  are  dying, 
and  the  community  as  a  whole  always  persists.  The  queen 
may  live  several  years,  perhaps  as  many  as  five.*  She  lays 
about  one  million  eggs  a  year. 

85.  The  ants. — There  are  many  species  of  ants,  two 
thousand  or  more,  and  all  of  them  live  in  communities  and 
show  a  truly  communal  life.  There  is  much  variety  of 
habit  in  the  lives  of  different  kinds  of  ants,  and  the  degree 
in  which  the  communal  or  social  life  is  specialized  or  elab- 
orated varies  much.  But  certain  general  conditions  pre- 
vail in  the  life  of  all  the  different  kinds  of  individuals — 


a 


FIG.  92.— Female  (a),  male  (ft),  and  worker  (c)  of  an  ant  (Camponotus  sp.). 

sexually  developed  males  and  females  that  possess  wings, 
and  sexually  undeveloped  workers  that  are  wingless  (Fig. 
92).  In  some  kinds  the  workers  show  structural  differ- 

*  A  queen  bee  has  been  kept  alive  for  fifteen  years. 


156  ANIMAL  LIFE 

ences  among  themselves,  being  divided  into  small  workers, 
large  workers,  and  soldiers.  The  workers  are  not,  as  with 
the  bees,  all  infertile  females,  but  they  are  both  male  and 
female,  both  being  infertile.  Although  the  life  of  the  ant 
communities  is  much  less  familiar  and  fully  known  than 
that  of  the  bees,  it  is  even  more  remarkable  in  its  speciali- 
zations and  elaborateness.  The  ant  home,  or  nest,  or  formi- 
cary, is,  with  most  species,  a  very  elaborate  underground, 
many-storied  labyrinth  of  galleries  and  chambers.  Certain 
rooms  are  used  for  the  storage  of  food ;  certain  others  as 
"  nurseries  "  for  the  reception  and  care  of  the  young ;  and 
others  as  stables  for  the  ants'  cattle,  certain  plant-lice  or 
scale-insects  which  are  sometimes  collected  and  cared  for  by 
the  ants.  The  food  of  ants  comprises  many  kinds  of  vege- 
table and  animal  substances,  but  the  favorite  food,  or  "  na- 
tional dish,"  as  it  has  been  called,  is  a  sweet  fluid  which  is 
produced  by  certain  small  insects,  the  plant-lice  (Aphidse) 
and  scale-insects  (Coccidae).  These  insects  live  on  the  sap 
of  plants  ;  rose-bushes  are  especially  favored  with  their  pres- 
ence. The  worker  ants  (and  we  rarely  see  any  ants  but 
the  wingless  workers,  the  winged  males  and  females  appear- 
ing out  of  the  nest  only  at  mating  time)  find  these  honey- 
secreting  insects,  and  gently  touch  or  stroke  them  with  their 
feelers  (antennae),  when  the  plant-lice  allow  tiny  drops  of 
the  honey  to  issue  from  the  body,  which  are  eagerly  drunk 
by  the  ants.  It  is  manifestly  to  the  advantage  of  the  ants 
that  the  plant-lice  should  thrive ;  but  they  are  soft-bodied, 
defenseless  insects,  and  readily  fall  a  prey  to  the  wander- 
ing predaceous  insects  like  the  lady-birds  and  aphis  lions. 
So  the  ants  often  guard  small  groups  of  plant-lice,  attack- 
ing, and  driving  away  the  would-be  ravagers.  When  the 
branch  on  which  the  plant-lice  are  gets  withered  and  dry, 
the  ants  have  been  observed  to  carry  the  plant-lice  care- 
fully to  a  fresh,  green  branch.  In  the  Mississippi  Valley  a 
certain  kind  of  plant-louse  lives  on  the  roots  of  corn.  Its 
eggs  are  deposited  in  the  ground  in  the  autumn  and  hatch 


ANIMAL  COMMUNITIES  AND  SOCIAL  LIFE         157 

the  following  spring  before  the  corn  is  planted.  Now,  the 
common  little  brown  ant  lives  abundantly  in  the  corn- 
fields, and  is  specially  fond  of  the  honey  secreted  by  the 
corn-root  plant-louse.  So,  when  the  plant-lice  hatch  in  the 
spring  before  there  are  corn  roots  for  them  to  feed  on,  the 
little  brown  ants  with  great  solicitude  carefully  place  the 
plant-lice  on  the  roots  of  a  certain  kind  of  knotweed  which 
grows  in  the  field,  and  protect  them  until  the  corn  ger- 
minates. Then  the  ants  remove  the  plant-lice  to  the  roots 
of  the  corn,  their  favorite  food  plant.  In  the  arid  lands  of 
New  Mexico  and  Arizona  the  ants  rear  their  scale-insects 
on  the  roots  of  cactus.  Other  kinds  of  ants  carry  plant- 
lice  into  their  nests  and  provide  them  with  food  there. 
Because  the  ants  obtain  food  from  the  plant-lice  and  take 
care  of  them,  the  plant-lice  are  not  inaptly  called  the  ants' 
cattle. 

Like  the  honey-bees,  the  young  ants  are  helpless  little 
grubs  or  larvae,  and  are  cared  for  and  fed  by  nurses.  The 
so-called  ants'  eggs,  little  white,  oval  masses,  which  we 
often  see  being  carried  in  the  mouths  of  ants  in  and  out  of 
an  ants'  nest,  are  not  eggs,  but  are  the  pupae  which  are 
being  brought  out  to  enjoy  the  warmth  and  light  of  the 
sun  or  being  taken  back  into  the  nest  afterward. 

In  addition  to  the  workers  that  build  the  nest  and  col- 
lect food  and  care  for  the  plant-lice,  there  is  in  many 
species  of  ants  a  kind  of  individuals  called  soldiers.  These 
are  wingless,  like  the  workers,  and  are  also,  like  the  work- 
ers, not  capable  of  laying  or  of  fertilizing  eggs.  It  is  the 
business  of  the  soldiers,  as  their  name  suggests,  to  fight. 
They  protect  the  community  by  attacking  and  driving 
away  predaceous  insects,  especially  other  ants.  The  ants 
are  among  the  most  warlike  of  insects.  The  soldiers  of  a 
community  of  one  species  of  ant  often  sally  forth  and 
attack  a  community  of  some  other  species.  If  successful 
in  battle  the  workers  of  the  victorious  community  take 
possession  of  the  food  stores  of  the  conquered  and  carry 


158  ANIMAL  LIFE 

them  to  their  own  nest.  Indeed,  they  go  even  further ;  they 
may  make  slaves  of  the  conquered  ants.  There  are  numer- 
ous species  of  the  so-called  slave-making  ants.  The  slave- 
makers  carry  into  their  own  nest  the  eggs  and  larvae  and 
pupae  of  the  conquered  community,  and  when  these  come 
to  maturity  they  act  as  slaves  of  the  victors — that  is,  they 
collect  food,  build  additions  to  the  nests,  and  care  for  the 
young  of  the  slave-makers.  This  specialization  goes  so  far 
in  the  case  of  some  kinds  of  ants,  like  the  robber-ant  of 
South  America  (Eciton),  that  all  of  the  Eciton  workers  have 
become  soldiers,  which  no  longer  do  any  work  for  them- 
selves. The  whole  community  lives,  therefore,  wholly  by 
pillage  or  by  making  slaves  of  other  kinds  of  ants.  There 
are  four  kinds  of  individuals  in  a  robber-ant  community — 
winged  males,  winged  females,  and  small  and  large  wing- 
less soldiers.  There  are  many  more  of  the  small  soldiers 
than  of  the  large,  and  some  naturalists  believe  that  the  few 
latter,  which  are  distinguished  by  heads  and  jaws  of  great 
size,  act  as  officers.  On  the  march  the  small  soldiers  are 
arranged  in  a  long,  narrow  column,  while  the  large  soldiers 
are  scattered  along  on  either  side  of  the  column  and  appear 
to  act  as  sentinels  and  directors  of  the  army.  The  obser- 
vations made  by  the  famous  Swiss  students  of  ants,  Huber 
and  Forel,  and  by  other  naturalists,  read  like  fairy  tales, 
and  yet  are  the  well-attested  and  often  reobserved  actual 
phenomena  of  the  extremely  specialized  communal  and 
social  life  of  these  animals. 

86.  Other  communal  insects. — The  termites  or  white  ants 
(not  true  ants)  are  communal  insects.  Some  species  of 
termites  in  Africa  live  in  great  mounds  of  earth,  often 
fifteen  feet  high.  The  community  comprises  hundreds  of 
thousands  of  individuals,  which  are  of  eight  kinds  (Fig  93), 
viz.,  sexually  active  winged  males,  sexually  active  winged 
females,  other  fertile  males  and  females  which  are  wingless, 
wingless  workers  of  both  sexes  not  capable  of  reproduc- 
tion, and  wingless  soldiers  of  both  sexes  also  incapable  of 


ANIMAL  COMMUNITIES  AND  SOCIAL  LIFE        159 

reproduction.  The  production  of  new  individuals  is  the 
sole  business  of  the  fertile  males  and  females  ;  the  workers 
build  the  nest  and  collect  food,  and  the  soldiers  protect  the 
community  from  the  attacks  of  marauding  insects.  The 
egg-laying  queen  grows  to  monstrous  size,  being  sometimes 


FIG.  93. — Termites,    a,  queen  ;  J,  male ;  c,  worker ;  d,  soldier. 

five  or  six  inches  long,  while  the  other  individuals  of  the 
community  are  not  more  than  half  or  three  quarters  of 
an  inch  long.  The  great  size  of  the  queen  is  due  to  the 
enormous  number  of  eggs  in  her  body. 

The  bumble-bees  live  in  communities,  but  their  social 
arrangements  are  very  simple  ones  compared  with  those  of 
the  honey-bee.  There  is,  in  fact,  among  the  bees  a  series 
of  gradations  from  solitary  to  communal  life.  The  inter- 
esting little  green  carpenter-bees  live  a  truly  solitary  life. 
Each  female  bores  out  the  pith  from  five  or  six  inches  of 
an  elder  branch  or  raspberry  cane,  and  divides  this  space 
into  a  few  cells  by  means  of  transverse  partitions  (Fig.  94). 
In  each  cell  she  lays  an  egg,  and  puts  with  it  enough  food 
— flower  pollen — to  last  the  grub  or  larva  through  its  life. 


160 


ANIMAL  LIFE 


She  then  waits  in  an  upper  cell  of  the  nest  until  the  young 
bees  issue  from  their  cells,  when  she  leads  them  off,  and 
each  begins  active  life  on  its  own  account.  The  mining- 


FIG.  94.— Nest  of  carpenter-bee. 


FIG.  95.— Nest  of  Andrena,  the  mining-bee. 


bees  (Andrena),  which  make  little  burrows  (Fig.  95)  in  a 
clay  bank,  live  in  large  colonies — that  is,  they  make  their 
nest  burrows  close  together  in  the  same  clay  bank,  but  each 
female  makes  her  own  burrow,  lays  her  own  eggs  in  it,  fur- 
nishes it  with  food — a  kind  of  paste  of  nectar  and  pollen — 
and  takes  no  further  care  of  her  young.  Nor  has  she  at 
any  time  any  special  interest  in  her  neighbors.  But  with 
the  smaller  mining-bees,  belonging  to  the  genus  Halictus^ 
several  females  unite  in  making  a  common  burrow,  after 
which  each  female  makes  side  passages  of  her  own,  extend- 


ANIMAL  COMMUNITIES  AND  SOCIAL  LIFE         161 


Q 


ing  from  the  main  or  public  entrance  burrow.  As  a  well- 
known  entomologist  has  said,  Andrena  builds  villages  com- 
posed of  individual  homes,  while  Halictus  makes  cities 
composed  of  apartment  houses.  The  bumble-bee  (Fig.  96), 
however,  establishes  a  real  community  with  a  truly  com- 
munal life,  although  a  very  simple  one.  The  few  bumble- 
bees which  we  see  in  winter  time  are  queens;  all  other 
bumble-bees  die  in  the  autumn.  In  the  spring  a  queen 
selects  some  deserted  nest  of  a  field-mouse,  or  a  hole  in 
the  ground,  gathers  pollen  which  she  molds  into  a  rather 
large  irregular  mass  and  puts  into 
the  hole,  and  lays  a  few  eggs  on  the 
pollen  mass.  The  young  grubs  or 
larvee  which  soon  hatch  feed  on  the 
pollen,  grow,  pupate,  and  issue  as 
workers — winged  bees  a  little  small- 
er than  the  queen.  These  workers 
bring  more  pollen,  enlarge  the  nest, 
and  make  irregular  cells  in  the  pol- 
len mass,  in  each  of  which  the  queen 
lays  an  egg.  She  gathers  no  more 
pollen,  does  no  more  work  except 
that  of  egg-laying.  From  these  new 
eggs  are  produced  more  workers,  and 
so  on  until  the  community  may  come 
to  be  pretty  large.  Later  in  the  sum- 
mer males  and  females  are  produced 
and  mate.  With  the  approach  of 
winter  all  the  workers  and  males  die, 
leaving  only  the  fertilized  females, 
the  queens,  to  live  through  the  win- 
ter and  found  new  communities  in 
the  spring. 

The  social  wasps  show  a  communal  life  like  that  of  the 
bumble-bees.     The  only  yellow-jackets  and  hornets  that 
live  through  the  winter  are  fertilized  females  or  queens. 
12 


FIG.  96.— Bumble-bees,  a, 
worker ;  6,  queen  or  fer- 
tile female. 


162 


ANIMAL  LIFE 


When  spring  comes  each  queen  builds  a  small  nest  sus- 
pended from  a  tree  branch,  and  consisting  of  a  small  comb 
inclosed  in  a  covering  or  envelope  open  at  the  lower  end. 
The  nest  is  composed  of  "  wasp  paper,"  made  by  chewing 
bits  of  weather-beaten  wood  taken  from  old  fences  or  out- 
buildings. In  each  of  the  cells  the  queen  lays  an  egg. 
She  deposits  in  the  cell  a  small  mass  of  food,  consisting  of 
some  chewed  insects  or  spiders.  From  these  eggs  hatch 
grubs  which  eat  the  food  prepared  for  them,  grow,  pupate, 
and  issue  as  worker  bees,  winged  and  slightly  smaller 
than  the  queen  (Fig.  97).  The  workers  enlarge  the  nest, 
adding  more  combs  and  making  many  cells,  in  each  of 
which  the  queen  lays  an  egg.  The  workers  provision  the 
cell  with  chewed  insects,  and  other  broods  of  workers  are 

rapidly  hatched.  The 
community  grows  in 
numbers  and  the  nest 
grows  in  size  until  it 
comes  to  be  the  great 
ball-like  oval  mass 
which  we  know  so  well 
as  a  hornets'  nest  (Figs. 
98  and  99),  a  thing  to  be 
left  untouched.  Some- 
times the  nest  is  built 
underground.  When 
disturbed,  they  swarm 
out  of  the  hole  and 
fiercely  attack  any  in- 
vading foe  in  sight. 
After  a  number  of 
broods  of  workers  has 
been  produced,  broods  of  males  and  females  appear  and 
mating  takes  place.  In  the  late  fall  the  males  and  all  of 
the  many  workers  die,  leaving  only  the  new  queens  to  live 
through  the  winter. 


FIG.  97.— The  yellow-jacket  (Vespa),  &  social 
wasp,    a,  worker  ;  b,  queen. 


ANIMAL  COMMUNITIES  AND  SOCIAL  LIFE         163 

The  bumble-bees  and  social  wasps  show  an  intermediate 
condition   between   the   simply   gregarious   or  neighborly 


FIG.  98. — Nest  of   Vespa,  a  social 
wasp.    From  photograph. 


FIG.  99.— Nest  of   Vespa   opened  to  show 
combs  within. 


mining-bees  and  the  highly  developed,  permanent  honey- 
bee community.  Naturalists  believe  that  the  highly  or- 
ganized communal  life  of  the  honey-bees  and  the  ants  is 
a  development  from  some  simple  condition  like  that  of  the 
bumble-bees  and  social  wasps,  which  in  its  turn  has  grown 
out  of  a  still  simpler,  mere  gregarious  assembly  of  the 
individuals  of  one  species.  It  is  not  difficult  to  see  how 
such  a  development  could  in  the  course  of  a  long  time  take 
place. 

87.  Gregariousness  and  mutual  aid. — The  simplest  form 
of  social  life  is  shown  among  those  kinds  of  animals  in 
which  many  individuals  of  one  species  keep  together,  form- 
ing a  great  band  or  herd.  In  this  case  there  is  not  much 
division  of  labor,  and  the  safety  of  the  individual  is  not 
wholly  bound  up  in  the  fate  of  the  herd.  Such  animals  are 


164  ANIMAL  LIFE 

said  to  be  gregarious  in  habit.  The  habit  undoubtedly  is 
advantageous  in  the  mutual  protection  and  aid  afforded 
the  individuals  of  the  band.  This  mutual  help  in  the  case 
of  many  gregarious  animals  is  of  a  very  positive  and  obvious 
character.  In  other  cases  this  gregariousness  is  reduced  to 
a  matter  of  slight  or  temporary  convenience,  possessing  but 
little  of  the  element  of  mutual  aid.  The  great  herds  of 
reindeer  in  the  north,  and  of  the  bison  or  buffalo  which 
once  ranged  over  the  Western  American  plains,  are  examples 
of  a  gregariousness  in  which  mutual  protection  from  ene- 
mies, like  wolves,  seems  to  be  the  principal  advantage  gained. 
The  bands  of  wolves  which  hunted  the  buffalo  show  the 
advantage  of  mutual  help  in  aggression  as  well  as  in  pro- 
tection. In  this  banding  together  of  wolves  there  is  active 
co-operation  among  individuals  to  obtain  a  common  food 
supply.  What  one  wolf  can  not  do — that  is,  tear  down  a 
buffalo  from  the  edge  of  the  herd— a  dozen  can  do,  and  all 
are  gainers  by  the  operation.  On  the  other  hand,  the  vast 
assembling  of  sea-birds  (Fig.  100)  on  certain  ocean  islands 
and  rocks  is  a  condition  probably  brought  about  rather  by 
the  special  suitableness  of  a  few  places  for  safe  breeding 
than  from  any  special  mutual  aid  afforded ;  still,  these  sea- 
birds  undoubtedly  combine  to  drive  off  attacking  eagles 
and  hawks.  Eagles  are  usually  considered  to  be  strictly 
solitary  in  habit  (the  unit  of  solitariness  being  a  pair,  not 
an  individual) ;  but  the  description,  by  a  Russian  naturalist, 
of  the  hunting  habits  of  the  great  white-tailed  eagle  (Hali- 
cetos  albicilla)  on  the  Eussian  steppes  shows  that  this  kind 
of  eagle  at  least  has  adopted  a  gregarious  habit,  in  which 
mutual  help  is  plainly  obvious.  This  naturalist  once  saw  an 
eagle  high  in  the  air,  circling  slowly  and  widely  in  perfect 
silence.  Suddenly  the  eagle  screamed  loudly.  "Its  cry 
was  soon  answered  by  another  eagle,  which  approached  it, 
and  was  followed  by  a  third,  a  fourth,  and  so  on,  till  nine 
or  ten  eagles  came  together  and  soon  disappeared."  The 
naturalist,  following  them,  soon  discovered  them  gathered 


ANIMAL  LIFE 

about  the  dead  body  of  a  horse.  The  food  found  by  the 
first  was  being  shared  by  all.  The  association  of  pelicans  in 
fishing  is  a  good  example  of  the  advantage  of  a  gregarious 
and  mutually  helpful  habit.  The  pelicans  sometimes  go 
fishing  in  great  bands,  and,  after  having  chosen  an  appro- 
priate place  near  the  shore,  they  form  a  wide  half-circle 
facing  the  shore,  and  narrow  it  by  paddling  toward  the 
land,  catching  the  fish  which  they  inclose  in  the  ever-nar- 
rowing circle. 

The  wary  Eocky  Mountain  sheep  (Fig.  101)  live  to- 
gether in  small  bands,  posting  sentinels  whenever  they 
are  feeding  or  resting,  who  watch  for  and  give  warning 
of  the  approach  of  enemies.  The  beavers  furnish  a  well- 
known  and  very  interesting  example  of  mutual  help,  and 
they  exhibit  a  truly  communal  life,  although  a  simple 
one.  They  live  in  "  villages  "  or  communities,  all  helping 
to  build  the  dam  across  the  stream,  which  is  necessary  to 
form  the  broad  marsh  or  pool  in  which  the  nests  or  houses 
are  built.  Prairie-dogs  live  in  great  villages  or  communi- 
ties which  spread  over  many  acres.  They  tell  each  other  by 
shrill  cries  of  the  approach  of  enemies,  and  they  seem  to 
visit  each  other  and  to  enjoy  each  other's  society  a  great 
deal,  although  that  they  afford  each  other  much  actual 
active  help  is  not  apparent.  Birds  in  migration  are  grega- 
rious, although  at  other  times  they  may  live  comparatively 
alone.  In  their  long  flights  they  keep  together,  often  with 
definite  leaders  who  seem  to  discover  and  decide  on  the 
course  of  flight  for  the  whole  great  flock.  The  wedge- 
shaped  flocks  of  wild  geese  flying  high  and  uttering  their 
sharp,  metallic  call  in  their  southward  migrations  are  well 
known  in  many  parts  of  the  United  States.  Indeed,  the 
more  one  studies  the  habits  of  animals  the  more  examples 
of  social  life  and  mutual  help  will  be  found.  Probably  most 
animals  are  in  some  degree  gregarious  in  habit,  and  in  all 
cases  of  gregariousness  there  is  probably  some  degree  of 
mutual  aid. 


FIG.  101.— Rocky  Mountain  or  bighorn  sheep.    By  permission  of  the 
publishers  of  Outing. 


168  ANIMAL  LIFE 

88.  Division  of  labor  and  basis  of  communal  life. — We  have 
learned  in  Chapters  II  and  IV  that  the  complexity  of  the 
bodies  of  the  higher  animals  depends  on  a  specialization  or 
differentiation  of  parts,  due  to  the  assumption  of  different 
functions  or  duties  by  different  parts  of  the  body ;  that  the 
degree  of  structural  differentiation  depends  on  the  degree 
or  extent  of  division  of  labor  shown  in  the  economy  of  the 
animal.  It  is  obvious  that  the  same  principle  of  division  of 
labor  with  accompanying  modification  of  structure  is  the 
basis  of  colonial  and  communal  life.  It  is  simply  a  mani- 
festation of  the  principle  among  individuals  instead  of 
among  organs.  The  division  of  the  necessary  labors  of  life 
among  the  different  zooids  of  the  colonial  jelly-fish  is  plain- 
ly the  reason  for  the  profound  and  striking,  but  always 
reasonable  and  explicable  modifications  of  the  typical  polyp 
or  medusa  body,  which  is  shown  by  the  swimming  zooids, 
the  feeding  zooids,  the  sense  zooids,  and  the  others  of  the 
colony.  And  similarly  in  the  case  of  the  termite  commu- 
nity, the  soldier  individuals  are  different  structurally  from 
the  worker  individuals  because  of  the  different  work  they 
have  to  do.  And  the  queen  differs  from  all  the  others,  be- 
cause of  the  extraordinary  prolificacy  demanded  of  her  to 
maintain  the  great  community. 

It  is  important  to  note,  however,  that  among  those  ani- 
mals that  show  the  most  highly  organized  or  specialized 
communal  or  social  life,  the  structural  differences  among 
the  individuals  are  the  least  marked,  or  at  least  are  not  the 
most  profound.  The  three  kinds  of  honey-bee  individuals 
differ  but  little;  indeed,  as  two  of  the  kinds,  male  and 
female,  are  to  be  found  in  the  case  of  almost  all  kinds  of 
animals,  whether  communal  in  habit  or  not,  the  only  unu- 
sual structural  specialization  in  the  case  of  the  honey  bee,  is 
the  presence  of  the  worker  individual,  which  differs  from 
the  usual  individuals  in  but  little  more  than  the  rudimen- 
tary condition  of  the  reproductive  glands.  Finally,  in  the 
case  of  man,  with  whom  the  communal  or  social  habit  is  so 


170  ANIMAL  LIFE 

all-important  as  to  gain  for  him  the  name  of  "  the  social 
animal,"  there  is  no  differentiation  of  individuals  adapted 
only  for  certain  kinds  of  work.  Among  these  highest 
examples  of  social  animals,  the  presence  of  an  advanced 
mental  endowment,  the  specialization  of  the  mental  power, 
the  power  of  reason,  have  taken  the  place  of  and  made 
unnecessary  the  structural  differentiation  of  individuals. 
The  honey-bee  workers  do  different  kinds  of  work :  some 
gather  food,  some  care  for  the  young,  and  some  make  wax 
and  build  cells,  but  the  individuals  are  interchangeable ; 
each  one  knows  enough  to  do  these  various  things.  There 
is  a  structural  differentiation  in  the  matter  of  only  one 
special  work  or  function,  that  of  reproduction. 

With  the  ants  there  is,  in  some  cases,  a  considerable 
structural  divergence  among  individuals,  as  in  the  genus 
Atta  of  South  America  with  six  kinds  of  individuals — 
namely,  winged  males,  winged  females,  wingless  soldiers, 
and  wingless  workers  of  three  distinct  sizes.  In  the  case 
of  other  kinds  with  quite  as  highly  organized  a  communal 
life  there  are  but  three  kinds  of  individuals,  the  winged 
males  and  females  and  the  wingless  workers.  The  workers 
gather  food,  build  the  nest,  guard  the  "  cattle  "  (aphids), 
make  war,  and  care  for  the  young.  Each  one  knows  enough 
to  do  all  these  various  distinct  things.  Its  body  is  not  so 
modified  that  it  can  do  but  one  kind  of  thing,  which  thing 
it  must  always  do. 

The  increase  of  intelligence,  the  development  of  the 
power  of  reasoning,  is  the  most  potent  factor  in  the  devel- 
opment of  a  highly  specialized  social  life.  Man  is  the 
example  of  the  highest  development  of  this  sort  in  the  ani- 
mal kingdom,  but  the  highest  form  of  social  development 
is  not  by  any  means  the  most  perfectly  communal. 

89.  Advantages  of  communal  life. — The  advantages  of 
communal  or  social  life,  of  co-operation  and  mutual  aid,  are 
real.  The  animals  that  have  adopted  such  a  life  are  among 
the  most  successful  of  all  animals  in  the  struggle  for  exist- 


ANIMAL  COMMUNITIES  AND  SOCIAL  LIFE 

ence.  The  termite  individual  is  one  of  the  most  defense- 
less, and,  for  those  animals  that  prey  on  insects,  one  of 
the  most  toothsome  luxuries  to  be  found  in  the  insect 
world.  But  the  termite  is  one  of  the  most  abundant  and 
widespread  and  successfully  living  insect  kinds  in  all  the 
tropics.  Where  ants  are  not,  few  insects  are.  The  honey- 
bee is  a  popular  type  of  a  successful  life.  The  artificial 
protection  afforded  the  honey-bee  by  man  may  aid  in  its 
struggle  for  existence,  but  it  gains  this  protection  because 
of  certain  features  of  its  communal  life,  and  in  Nature  the 
honey-bee  takes  care  of  itself  well.  The  Little  Bee  People 
of  Kipling's  Jungle  Book,  who  live  in  great  communities  in 
the  rocks  of  Indian  hills,  can  put  to  rout  the  largest  and 
fiercest  of  the  jungle  animals.  Co-operation  and  mutual 
aid  are  among  the  most  important  factors  which  help  in 
the  struggle  for  existence.  Its  great  advantages  are,  how- 
ever, in  some  degree  balanced  by  the  fact  that  mutual  help 
brings  mutual  dependence.  The  community  or  society  can 
accomplish  greater  things  than  the  solitary  individuals,  but 
co-operation  limits  freedom,  and  often  sacrifices  the  indi- 
vidual to  the  whole. 


CHAPTER  X 

COMMENSALISM   AND   SYMBIOSIS 

90.  Association  between  animals  of  different  species. — The 
living  together  and  mutual  help  discussed  in  the  last  chap- 
ter concerned  in  each  instance  a  single  species  of  animal. 
All  the  various  members  of  a  pack  of  wolves  or  of  a  com- 
munity of  ants  are  individuals  of  the  same  species.  But 
there  are  many  instances  of  an  association  of  individuals 
of  different  kinds  of  animals.  The  number  of  individuals 
concerned,  however,  is  usually  but  two — that  is,  one  of 
each  of  the  two  kinds  of  animals.  In  many  cases  of  an 
association  of  individuals  of  different  species  one  kind 
derives  great  benefit  and  the  other  suffers  more  or  less 
injury  from  the  association.  One  kind  lives  at  the  expense 
of  the  other.  This  association  is  called  parasitism,  and  is 
discussed  in  the  next  chapter.  In  some  cases,  however, 
neither  kind  of  animal  suffers  from  the  presence  of  the 
other.  The  two  live  together  in  harmony  and  presumably 
to  their  mutual  advantage.  In  some  cases  this  mutual 
advantage  is  obvious.  This  kind  of  association  is  called 
commensalism  or  symbiosis.  The  term  commensalism  may 
be  used  to  denote  a  condition  where  the  two  animals  are 
not  so  intimately  associated  nor  derive  such  obvious  mu- 
tual advantage  from  the  association,  as  in  that  condition 
of  very  intimate  and  permanent  association  with  obvious 
co-operative  and  marked  advantage  that  may  be  called 
symbiosis.  A  few  examples  of  each  of  these  interesting 
conditions  of  association  between  which  it  is  impossible  to 
make  any  sharp  distinction,  will  be  given. 
172 


COMMENSAL1SM  AND  SYMBIOSIS  173 

91.  Commensalism. — A  curious  example  of  commensalism 
is  afforded  by  the  different  species  of  Remoras  (Echenididce) 
which  attach  themselves  to  sharks,  barracudas,  and  other 
large  fishes  by  means  of  a  sucking  disk  on  the  top  of  the 
head  (Fig.  103).  This  disk  is  made  by  a  modification  of 


PIG.  103.— Remora,  with  dorsal  fin  modified  to  be  a  sucking  plate  by  which  the 
fish  attaches  itself  to  a  shark. 

the  dorsal  fin.  The  Eemora  thus  attached  to  a  shark  may 
be  carried  about  for  weeks,  leaving  its  host  only  to  secure 
food.  This  is  done  by  a  sudden  dash  through  the  water. 
The  Remora  injures  the  shark  in  no  way  save,  perhaps,  by 
the  slight  check  its  presence  gives  to  ths  shark's  speed  in 
swimming. 

Whales,  similarly,  often  carry  barnacles  about  with 
them.  These  barnacles  are  permanently  attached  to  the 
skin  of  the  whale  just  as  they  would  be  to  a  stone  or 
wooden  pile.  Many  small  crustaceans,  annelids,  mollusks, 
and  other  invertebrates  burrow  into  the  substance  of  living 
sponges,  not  for  the  purpose  of  feeding  on  them,  but  for 
shelter.  On  the  other  hand,  the  little  boring  sponge 
( Cliona)  burrows  in  the  shells  of  oysters  and  other  bivalves 
for  protection.  These  are  hardly  true  cases  of  even  that 
lesser  degree  of  mutually  advantageous  association  which 
we  are  calling  commensalism.  But  some  species  of  sponge 
"  are  never  found  growing  except  on  the  backs  or  legs  of 
certain  crabs."  In  these  cases  the  sponge,  with  its  many 
plant-like  branches,  protects  the  crab  by  concealing  it  from 
its  enemies,  while  the  sponge  is  benefited  by  being  carried 
about  by  the  crab  to  new  food  supplies.  Certain  sponges 


ANIMAL  LIFE 

and  polyps  are  always  found  growing  in  close  association, 
though  what  the  mutual  advantage  of  this  association  is 
has  not  yet  been  found  out. 

Among  the  coral  reefs  near  Thursday  Island  (between 
New  Guinea  and  Australia)  there  lives  an  enormous  kind 
of  sea-anemone  or  polyp.  Individuals  of  this  great  polyp 
measure  two  feet  across  the  disk  when  fully  expanded. 
In  the  interior,  the  stomach  cavity,  which  communicates 
freely  with  the  outside,  by  means  of  the  large  mouth  open- 
ing at  the  free  end  of  the  polyp,  there  may  often  be  found 
a  small  fish  (Amphiprion  percula).  That  this  fish  is  pur- 
posely in  the  gastral  cavity  of  the  polyp  is  proved  by  the 
fact  that  when  it  is  dislodged  it  invariably  returns  to  its 
singular  lodging-place.  The  fish  is  brightly  colored,  being 
of  a  brilliant  vermilion  hue  with  three  broad  white  cross 
bands.  The  discoverer  of  this  peculiar  habit  suggests  that 
there  are  mutual  benefits  to  fish  and  polyp  from  this  habit. 
"  The  fish  being  conspicuous,  is  liable  to  attacks,  which  it 
escapes  by  a  rapid  retreat  into  the  sea-anemone ;  its  enemies 
in  hot  pursuit  blunder  against  the  outspread  tentacles  of 
the  anemone  and  are  at  once  narcotized  by  the  'thread 
cells '  shot  out  in  innumerable  showers  from  the  tentacles, 
and  afterward  drawn  into  the  stomach  of  the  anemone  and 
digested." 

Small  fish  of  the  genus  Nomeus  may  often  be  found 
accompanying  the  beautiful  Portuguese  man-of-war  (Phy- 
salia)  as  it  sails  slowly  about  on  the  ocean's  surface  (Fig. 
104).  These  little  fish  lurk  underneath  the  float  and 
among  the  various  hanging  thread-like  parts  of  the  Phy- 
salia,  which  are  provided  with  stinging  cells.  The  fish  are 
protected  from  their  enemies  by  their  proximity  to  these 
stinging  threads,  but  of  what  advantage  to  the  man-of- 
war  their  presence  is  is  not  understood.  Similarly,  several 
kinds  of  medusae  are  known  to  harbor  or  to  be  accompanied 
by  young  or  small  adult  fishes. 

In  the  nests  of  the  various  species  of  ants  and  termites 


COMMENSALISM  AND  SYMBIOSIS 


175 


many  different  kinds  of  other  insects  have  been  found. 
Some  of  these  are  harmful  to  their  hosts,  in  that  they  feed 
on  the  food  stores  gathered  by  the  industrious  and  provi- 
dent ant,  but  others  appear 
to  feed  only  on  refuse  or  use- 
less substances  in  the  nest. 
Some  may  even  be  of  help  to 
their  hosts.  Over  one  thou- 
sand species  of  these  rryn-rie- 
cophilous  (ant -loving)  and 
termitophilous  (termite -lov- 
ing) insects  have  been  re- 
corded by  collectors  as  living 
habitually  in  the  nests  of  ants 
and  termites.  The  owls  and 
rattlesnakes  which  live  with 
the  prairie-dogs  in  their  vil- 
lages afford  a  familiar  exam- 
ple of  commensalism. 

92.  Symbiosis. — Of  a  more 
intimate  character,  and  of 
more  obvious  and  certain  mu- 
tual advantage,  is  the  well- 
known  case  of  the  symbiotic 
association  of  some  of  the 
numerous  species  of  hermit- 
crabs  and  certain  species  of 
sea-anemones.  The  hermit- 
crab  always  takes  for  his 
habitation  the  shell  of  an- 
other animal,  often  that  of 
the  common  whelk.  All  of 

the  hind  part  of  the  crab  lies  inside  the  shell,  while  its 
head  with  its  great  claws  project  from  the  opening  of  the 
shell.  On  the  surface  of  the  shell  near  the  opening  there 
is  often  to  be  found  a  sea-anemone,  or  sea-rose  (Fig.  105). 


FIG.  104.— A  Portuguese  man-of-war 
(Physalia),  with  man-of-war  fishes 
(Nomeus  gronovii)  living  in  the 
shelter  of  the  stinging  feelers. 
Specimens  from  off  Tampa,  Fla. 


ANIMAL  LIFE 

This  sea-anemone  is  fastened  securely  to  the  shell,  and  has 
its  mouth  opening  and.  tentacles  near  the  head  of  the  crab. 
The  sea-anemone  is  carried  from  place  to  place  by  the  her- 
mit-crab, and  in  this  way  is  much  aided  in  obtaining  food. 
On  the  other  hand,  the  crab  is  protected  from  its  enemies 
by  the  well-armed  and  dangerous  tentacles  of  the  sea-anem- 


Pie.  105.— Hermit-crab  (Pagurus)  in  shell,  with  a  sea-anemone  (Adamsia  palliata, 
attached  to  the  shell.— After  HERTWIG. 


one.  In  the  tentacles  there  are  many  thousand  long, 
slender  stinging  threads,  and  the  fish  that  would  obtain 
the  hermit-crab  for  food  must  first  deal  with  the  stinging 
anemone.  There  is  no  doubt  here  of  the  mutual  advan- 
tage gained  by  these  two  widely  different  but  intimately 
associated  companions.  If  the  sea-anemone  be  torn  away 
from  the  shell  inhabited  by  one  of  these  crabs,  the  crab 
will  wander  about,  carefully  seeking  for  another  anemone. 
When  he  finds  it  he  struggles  to  loosen  it  from  its  rock 
or  from  whatever  it  may  be  growing  on,  and  does  not  rest 
until  he  has  torn  it  loose  and  placed  it  on  his  shell. 

There  are  numerous  small  crabs  called  pea-crabs  (Pin- 
notheres) which  live  habitually  inside  the  shells  of  living 


COMMENSALISM  AND  SYMBIOSIS  177 

mussels.     The  mussels  and  the  crabs  live  together  in  per- 
fect harmony  and  to  their  mutual  benefit. 

There  are  a  few  extremely  interesting  cases  of  symbiosis 
in  which  not  different  kinds  of  animals  are  concerned,  but 
animals  and  plants.  It  has  long  been  known  that  some 
sea-anemones  pos- 
sess certain  body 
cells  which  con- 
tain chlorophyll, 
that  green  sub- 
stance character- 
istic of  the  green 
plants,  and  only 
in  few  cases  pos- 
sessed by  animals. 
When  these  chlo- 
rophyll -bearing 
sea-anemones  were 
first  found,  it  was 
believed  that  the 
chlorophyll  cells 

,,     *    ,  ,    ,       FIG.  106.— The   crab   Epizoanthus  paguriphilus,  with 

really  belonged   to          the  sea-anemone  Parapagurus  pilosiramus  on  its 

the  animal's  body,  shell, 
and  that  this  con- 
dition broke  down  one  of  the  chiefest  and  most  readily 
apparent  distinctions  between  animals  and  plants.  But 
it  is  now  known  that  these  chlorophyll-bearing  cells  are 
microscopic,  one-celled  plants,  green  algae,  which  live  ha- 
bitually in  the  bodies  of  the  sea-anemone.  It  is  a  case 
of  true  symbiosis.  The  algae,  or  plants,  use  as  food  the 
carbonic-acid  gas  which  is  given  off  in  the  respiratory 
processes  of  the  sea-anemone,  and  the  sea-anemone  breathes 
in  the  oxygen  given  off  by  the  algae  in  the  process  of  ex- 
tracting the  carbon  for  food  from  the  carbonic-acid  gas. 
These  algae,  or  one-celled  plants,  lie  regularly  only  in  the 
innermost  of  the  three  cell  layers  which  compose  the  wall 
13 


178 


ANIMAL  LIFE 


or  body  of  the  sea-anemone  (Fig.  107).      They  penetrate 
into  and  lie  in  the  interior  of  the  cells  of  this  layer  whose 
special  function  is  that  of  digestion.    They  give  this  inner- 
most layer  of  cells 
a     distinct     green 
color. 

There  are  other 
examples  known  of 
the  symbiotic  asso- 
ciation of  plants 
and  animals ;  and 
if  we  were  to  fol- 
low the  study  of 
symbiosis  into  the 
plant  kingdom  we 


FIG.  107.— Diagrammatic  section  of  sea-anemone,  a, 
the  inner  cell  layer  containing  alga  cells,  the  two 
isolated  cells  at  right  being  cells  of  this  layer  with 
contained  algae;  b,  middle  body  wall  layer;  c,  outer 
body  wall  layer.— After  HERTWIG. 


should  find  that  in 
one  of  the  large 
groups  of  plants, 
the  familiar  lichens 
which  grow  on 

rocks  and  tree  trunks  and  old  fences,  every  member  lives 
symbiotically.  A  lichen  is  not  a  single  plant,  but  is  always 
composed  of  two  plants,  an  alga  (chlorophyll-bearing)  and 
a  fungus  (without  chlorophyll)  living  together  in  a  most 
intimate,  mutually  advantageous  association. 


CHAPTER  XI 

PARASITISM    AND   DEGENERATION 

93.  Eelation  of  parasite  and  host.— In  addition  to  the  vari- 
ous ways  of  living  together  of  animals  already  described, 
namely,  the  social  life  of  individuals  of  a  single  species  and 
the  commensal  and  symbiotic  life  of  individuals  of  differ- 
ent species,  there  is  another  kind  of  association  among  ani- 
mals that  is  very  common.  In  cases  of  symbiosis  the  two 
animals  living  together  are  of  mutual  advantage  to  each 
other ;  both  profit  by  the  association.  But  there  are  many 
instances  in  the  animal  kingdom  of  an  association  between 
two  animals  by  which  one  gains  advantages  great  or  small, 
sometimes  even  obtaining  all  the  necessities  of  life,  while 
the  other  gains  nothing,  but  suffers  corresponding  disad- 
vantage, often  even  the  loss  of  life  itself.  This  is  the  asso- 
ciation of  parasite  and  host ;  the  relation  between  two  ani- 
mals whereby  one,  the  parasite,  lives  on  or  in  the  other,  the 
host,  and  at  the  expense  of  the  host.  Parasitism  is  a  com- 
mon phenomenon  in  all  groups  of  animals,  although  the 
parasites  themselves  are  for  the  most  part  confined  to  the 
classes  of  invertebrates.  Among  the  simplest  animals  or 
Protozoa  there  are  parasites,  as  Gregarina,  which  lives  in 
the  bodies  of  insects  and  crustaceans ;  there  are  parasitic 
worms,  and  parasitic  crustaceans  and  mollusks  and  insects, 
and  a  few  vertebrates.  When  an  animal  can  get  along 
more  safely  or  more  easily  by  living  at  the  expense  of  some 
other  animal  and  takes  up  such  a  life,  it  becomes  a  parasite. 
Parasitism  is  naturally,  therefore,  not  confined  to  any  one 
group  or  class  of  animals. 

179 


180  ANIMAL  LIFE 

94.  Kinds  of  parasitism,— The  bird-lice  (Mallophaga), 
which  infest  the  bodies  of  all  kinds  of  birds  and  are  found 
especially  abundant  on  domestic  fowls,  live  upon  the  out- 
side of  the  bodies  of  their  hosts,  feeding  upon  the  feathers 
and  dermal  scales.  They  are  examples  of  external  parasites. 
Other  examples  are  fleas  and  ticks,  and  the  crustaceans  called 
fish-lice  and  whale-lice,  which  are  attached  to  marine  ani- 
mals. On  the  other  hand,  almost  all  animals  are  infested  by 
certain  parasitic  worms  which  live  in  the  alimentary  canal, 
like  the  tape-vrorm,  or  imbedded  in  the  muscles,  like  the 
trichina.  These  are  examples  of  internal  parasites.  Such 
parasites  belong  mostly  to  the  class  of  worms,  and  some  of 
them  are  very  injurious,  sucking  the  blood  from  the  tissues 
of  the  host,  while  others  feed  solely  on  the  partly  digested 
food.  There  are  also  parasites  that  live  partly  within  and 
partly  on  the  outside  of  the  body,  like  the  Sacculina,  which 
lives  on  various  kinds  of  crabs.  The  body  of  the  Sacculina 
consists  of  a  soft  sac  which  lies  on  the  outside  of  the  crab's 
body,  and  of  a  number  of  long,  slender  root-like  processes 
which  penetrate  deeply  into  the  crab's  body,  and  take  up 
nourishment  from  within.  The  Sacculina  is  itself  a  crus- 
tacean or  crab-like  creature.  The  classification  of  para- 
sites as  external  and  internal  is  purely  arbitrary,  but  it  is 
often  a  matter  of  convenience. 

Some  parasites  live  for  their  whole  lifetime  on  or  in  the 
body  of  the  host,  as  is  the  case  with  the  bird-lice.  Their 
eggs  are  laid  on  the  feathers  of  the  bird  host ;  the  young 
when  hatched  remain  on  the  bird  during  growth  and  devel- 
opment, and  the  adults  only  rarely  leave  the  body,  usually 
never.  These  may  be  called  permanent  parasites.  On  the 
other  hand,  fleas  leap  off  or  on  a  dog  as  caprice  dictates ; 
or,  as  in  other  cases,  the  parasite  may  pass  some  definite 
part  of  its  life  as  a  free,  non-parasitic  organism,  attaching 
itself,  after  development,  to  some  animal,  and  remaining 
there  for  the  rest  of  its  life.  These  parasites  may  be  called 
temporary  parasites.  But  this  grouping  or  classification, 


PARASITISM  AND  DEGENERATION 


183 


protozoan  is  as  simple  as  an  animal's  body  can  be,  being 
composed  of  but  a  single  cell,  degeneration  can  not  occur 
in  the  cases  of  these  parasites.  There  are,  besides  Grega- 
rina,  numerous  other  parasitic  one-celled  animals,  several 
kinds  living  inside  the  cells  of  their  host's  body.  One 
kind  lives  in  the  blood-corpuscles  of  the  frog,  and  another 
in  the  cells  of  the  liver  of  the  rabbit. 

97.  The  tape-worm  and  other  flat-worms. — In  the  great 
group  of  flat-worms  (Platyhelminthes),  that  group  of  ani- 
mals which  of  all  the  principal  animal  groups  is  widest 
in  its  distribution,  perhaps  a  major- 
ity of  the  species  are  parasites.  In- 
stead of  being  the  exception,  the 
parasitic  life  is  the  rule  among  these 
worms.  Of  the  three  classes  into 
which  the  flat -worms  are  divided 
almost  all  of  the  members  of  two  of 
the  classes  are  parasites.  The  com- 
mon tape-worm  (Tcenia)  (Fig.  108), 
which  lives  parasitically  in  the  intes- 
tine of  man,  is  a  good  example  of 
one  of  these  classes.  "  It  has  the 
form  of  a  narrow  ribbon,  which  may 
attain  the  length  of  several  yards, 
attached  at  one  end  to  the  wall  of 
the  intestine,  the  remainder  hanging 
freely  in  the  interior."  Its  body  is 
composed  of  segments  or  serially 
arranged  parts,  of  which  there  are 

about  eight  hundred  and  fifty  altogether.  It  has  no  mouth 
nor  alimentary  canal.  It  feeds  simply  by  absorbing  into 
its  body,  through  the  surface,  the  nutritious,  already  di- 
gested liquid  food  in  the  intestine.  There  are  no  eyes 
nor  other  special  sense  organs,  nor  any  organs  of  locomo- 
tion. The  body  is  very  degenerate.  The  life  history  of 
the  tape-worm  is  interesting,  because  of  the  necessity  of 


FIG.  108.— Tape-worm  (Tcenia 
solium).  In  upper  left- 
hand  corner  of  figure  the 
head  much  magnified.  — 
After  LETJCKABT. 


184 


ANIMAL  LIFE 


two  hosts  for  its  completion.  The  eggs  of  the  tape-worm 
pass  from  the  intestine  with  the  excreta,  and  must  be 
taken  into  the  body  of  some  other  animal  in  order  to  de- 
velop. In  the  case  of  one  of  the  several  species  of  tape- 
worms that  infest  man  this  other  host  must  be  the  pig. 
In  the  alimentary  canal  of  the  pig  the  young  tape-worm 
develops,  and  later  bores  its  way  through  the  walls  of  the 
canal  and  becomes  imbedded  in  the  muscles.  There  it  lies, 
until  it  finds  its  way  into  the  alimentary  canal  of  man  by 
his  eating  the  flesh  of  the  pig.  In  the  intestine  of  man 
the  tape-worm  continues  to  develop 
until  it  becomes  full  grown. 

In  a  lake  in  Yellowstone  Park 
the  suckers  are  infested  by  one  of 
the  flat-worms  (Ligula)  that  at- 
tains a  size  of  nearly  one  fourth 
the  size  of  the  fish  in  whose  in- 
testines it  lives.  If  the  tape-worm 
of  man  attained  such  a  compara- 
tive size,  a  man  of  two  hundred 
pounds'  weight  would  be  infested  by 
a  parasite  of  fifty  pounds'  weight. 

98.  Trichina  and  other  round- 
worms. — Another  group  of  animals, 
many  of  whose  numbers  are  para- 
sites, are  the  round-worms  or  thread- 
worms (Kemathelminthes).  The 
free-living  round-worms  are  active, 
well  -  organized  animals,  but  the 
parasitic  kinds  all  show  a  greater 
or  less  degree  of  degeneration.  One 

of  the  most  terrible  parasites  of  man  is  a  round-worm  called 
Trichina  spiralis  (Fig.  109).  It  is  a  minute  worm,  from 
one  to  three  millimetres  long,  which  in  its  adult  condition 
lives  in  the  intestine  of  man  or  of  the  pig  or  other  mam- 
mals. The  young  are  born  alive  and  bore  through  the  walls. 


FIG.  109.  —  Trichina  spiralis 
(after  CLAUS).  a,  male  ;  b, 
encysted  form  in  muscle  ;  c, 
female. 


PARASITISM  AND  DEGENERATION  185 

of  the  intestine.  They  migrate  to  the  voluntary  muscles 
of  the  hosts,  especially  those  of  the  limbs  and  back,  and 
here  each  worm  coils  itself  up  in  a  muscle  fiber  and  be- 
comes inclosed  in  a  spindle-shaped  cyst  or  cell  (Fig.  109,  #). 
A  single  muscle  may  be  infested  by  hundreds  of  thousands  of 
these  minute  worms.  It  has  been  estimated  that  fully  one 
hundred  million  encysted  worms  have  existed  in  the  mus- 
cles of  a  "  trichinized  "  human  body.  The  muscles  undergo 
more  or  less  degeneration,  and  the  death  of  the  host  may 
occur.  It  is  necessary,  for  the  further  development  of  the 
worms,  that  the  flesh  of  the  host  be  eaten  by  another  mam- 
mal, as  the  flesh  of  the  pig  by  man,  or  the  flesh  of  man  by 
a  pig  or  rat.  The  Trichince  in  the  alimentary  canal  of 
the  new  host  develop  into  active  adult  worms  and  produce 
new  young. 

In  the  Yellowstone  Lake  the  trout  are  infested  by  the 
larvae  or  young  of  a  round-worm  (Bothriocephalus  cordiceps) 
which  reaches  a  length  of  twenty  inches,  and  which  is 
often  found  stitched,  as  it  were,  through  the  viscera  and 
the  muscles  of  the  fish.  The  infested  trout  become  feeble 
and  die,  or  are  eaten  by  the  pelicans  which  fish  in  this 
lake.  In  the  alimentary  canal  of  the  pelican  the  worms 
become  adult,  and  parts  of  the  worms  containing  eggs 
escape  from  the  alimentary  canal  with  the  excreta.  These 
portions  of  worms  are  eaten  by  the  trout,  and  the  eggs  give 
birth  to  new  worms  which  develop  in  the  bodies  of  the 
fish  with  disastrous  effects.  It  is  estimated  that  for  each 
pelican  in  Yellowstone  Lake  over  five  million  eggs  of  the 
parasitic  worms  are  discharged  into  the  lake. 

The  young  of  various  carnivorous  animals  are  often 
infested  by  one  of  the  species  of  round-worms  called  "  pup- 
worms  "  ( Untinaria).  Eecent  investigations  show  that 
thousands  of  the  young  or  pup  fur-seals  are  destroyed  each 
year  by  these  parasites.  The  eggs  of  the  worm  lie  through 
the  winter  in  the  sands  of  the  breeding  grounds  of  the  fur- 
seal.  The  young  receive  them  from  the  fur  of  the  mother 


I 


PARASITISM  AND  DEGENERATION  187 

and  the  worm  develops  in  the  upper  intestine.  It  feeds  on 
the  blood  of  the  young  seal,  which  finally  dies  from  anaemia. 
On  the  beaches  of  the  seal  islands  in  Bering  Sea  there  are 
sometimes  hundreds  of  dead  seal  pups  which  have  been 
killed  by  this  parasite  (Fig.  110). 

99.  Sacculina. — Among  the  more  highly  organized  ani- 
mals the  results  of  a  parasitic  life,  in  degree  of  structural 
degeneration,  can  be  more  readily  seen.  A  well-known  para- 
site, belonging  to  the  Crustacea — the  class  of  shrimps,  crabs, 
lobsters,  and  cray-fishes — is  Sacculina.  The  young  Sac- 
culina is  an  active,  free-swimming  larva  much  like  a  young 
prawn  or  young  crab.  But  the  adult  bears  absolutely  no 
resemblance  to  such  a  typical  crustacean  as  a  cray-fish  or 
crab.  The  Sacculina  after  a  short  period  of  independent 
existence  at- 
taches itself  to 
the  abdomen  of 
a  crab,  and 
there  completes 
its  develop- 
ment  while  liv- 
ing  as  a  para- 
site.  In  its 
adult  condition 
(Fig.  Ill)  it  is 
simply  a  great 

i  -]•-!  FIG.  111. — Sacculina,  &  crustacean  parasite  of  crabs,    a,  at- 

ac»  tached  to  a  crab,  with  root-like  processes  penetrating  the 

bearing       many  crab's  body  ;  b,  removed  from  the  crab. 

delicate    r  o  o  t- 

like  suckers  which  penetrate  the  body  of  the  crab  host  and 
absorb  nutriment.  The  Sacculina  has  no  eyes,  no  mouth 
parts,  no  legs,  or  other  appendages,  and  hardly  any  of  the 
usual  organs  except  reproductive  organs.  Degeneration 
here  is  carried  very  far. 

Other  parasitic   Crustacea,  as  the  numerous  kinds  of 
fish-lice  (Fig.  112)  which  live  attached  to  the  gills  or  to 


188 


ANIMAL  LIFE 


other  parts  of  fish,  and  derive  all  their  nutriment  from  the 
body  of  the  fish,  show  various  degrees  of  degeneration.  With 
some  of  these  fish-lice  the  female, 
which  looks  like  a  puffed-out  worm, 
is  attached  to  the  fish  or  other  aquatic 
animal,  while  the  male,  which  is  per- 
haps only  a  tenth  of  the  size  of  the 
female,  is  permanently  attached  to 
the  female,  living  parasitically  on  her. 
100.  Parasitic  insects,  —  Among 
the  insects  there  are  many  kinds 
that  live  parasitically  for  part  of 
their  life,  and  not  a  few  that  live  as 
FIG.  112. -Fish-louse  (Ler-  parasites  for  their  whole  life.  The 

nceocera).   a,  adult ;  b,  larva.  ,  .         _.         ,  , -,0\          T    ,-, 

true  sucking  lice  (Fig.  113)  and  the 

bird-lice  (Fig.  114)  live  for  their  whole  lives  as  external 
parasites  on  the  bodies  of  their  host,  but  they  are  not 
fixed  —  that    is,  they   retain 
their  legs  and  power  of  loco- 
motion, although  they  have 
lost  their  wings  through  de- 
generation.    The  eggs  of  the 
lice  are  deposited  on  the  hair 
of  the  mammal  or  bird  that 


FIG.  113.— Sucking  louse  (Pediculm)  of 
human  body. 


FIG.  114.— Bird  louse  (Lipeurus  densus). 


serves  as  host ;  the  young  hatch  and  immediately  begin  to 
live  as  parasites,  either  sucking  the  blood  or  feeding  on  the 


PARASITISM  AND  DEGENERATION  189 

hair  or  feathers  of  the  host.  In  the  order  Hymenoptera 
there  are  several  families,  all  of  whose  members  live  during 
their  larval  stage  as  parasites.  We  may  call  all  these  hy- 
menopterous  parasites  ichneumon  flies.  The  ichneumon 
flies  are  parasites  of  other  insects,  especially  of  the  larvae  of 
beetles  and  moths  and  butterflies.  In  fact,  the  ichneumon 
flies  do  more  to  keep  in  check  the  increase  of  injurious  and 
destructive  caterpillars  than  do  all  our  artificial  remedies 
for  these  insect  pests.  The  adult  ichneumon  fly  is  four- 
winged  and  lives  an  active,  independent  life.  It  lays  its 
eggs  either  in  or  on  or  near  some  caterpillar  or  beetle  grub, 
and  the  young  ichneumon,  when  hatched,  burrows  about  in 
the  body  of  its  host,  feeding  on  its  tissues,  but  not  attacking 
such  organs  as  the  heart  or  nervous  ganglia,  whose  injury 
would  mean  immediate  death  to  the  host.  The  caterpillar 
lives  with  the  ichneumon  grub  within  it,  usually  until  nearly 


FIG.  115.— Parasitized  caterpillar  from  which  the    ichneumon    fly   parasites   have 
issued,  showing  the  circular  holes  of  exit  in  the  skin. 

time  for  its  pupation.  In  many  instances,  indeed,  it  pu- 
pates, with  the  parasite  still  feeding  within  its  body,  but  it 
never  comes  to  maturity.  The  larval  ichneumon  fly  pupates 
either  within  the  body  of  its  host  (Fig.  115)  or  in  a  tiny 
silken  cocoon  outside  of  its  body  (Fig.  116).  From  the 
cocoons  the  adult  winged  ichneumon  flies  emerge,  and 
after  mating  find  another  host  on  whose  body  to  lay  their 
eggs. 

One  of  the  most  interesting  ichneumon  flies  is  Thalessa 
(Fig.  119),  which  has  a  remarkably  long,  slender,  flexible 
ovipositor,  or  egg-laying  organ.  An  insect  known  as  the 


190 


ANIMAL  LIFE 


pigeon  horn-tail  (Tremex  columla)  (Fig.  117)  deposits  its 
eggs,  by  means  of  a  strong,  piercing  ovipositor,  half  an  inch 
deep  in  the  trunk  wood  of  growing  trees.  The  young  or 


FIG.  116.— Caterpillar  with  cocoons  of  the  pupae  of  ichneumon  fly  parasites,  and 
(above)  one  of  the  adult  ichneumon  flies.    The  lines  indicate  natural  dimensions. 

larval  Tremex  is  a  soft-bodied  white  grub,  which  bores 
deeply  into  the  trunk  of  the  tree,  filling  up  the  burrow  be- 
hind it  with  small  chips.  The  Thalessa  is  a  parasite  of  the 
Tremex,  and  "  when  a  female  Thalessa  finds  a  tree  infested 
by  Tremex,  she  selects  a  place  which  she  judges  is  opposite 


PARASITISM  AND  DEGENERATION 


191 


a  Tremex  burrow,  and,  elevating  her  long  ovipositor  in  a 
loop  over  her  back,  with  its  tip  on  the  bark  of  the  tree  (Fig. 


FIG.  117.— The  pigeon  horn-tail  (Tremex 
columbd),  with  strong  boring  ovipositor. 


FIG.  lia—  Thalessa  lunator  boring— After 

COMSTOCK. 


Fio.  119.— The  large  ichneumon  fly 
Thalessa,  with  long  flexible  oviposi- 
tor. The  various  parts  of  this  ovi- 
positor are  spread  apart  in  the  fig- 
ure ;  naturally  they  lie  together  to 
form  a  single  piercing  organ. 


118),  she  makes  a  derrick  out 

of  her  body  and  proceeds  with 

great  skill  and  precision  to  drill  a  hole  into  the  tree.    When 

the   Tremex  burrow  is  reached  she  deposits  an  egg  in  it. 


192 


ANIMAL  LIFE 


FIG.  120.— Wasp  (Polistes),  with  female  Stylops  para- 
site (a?)  in  body. 


The  larva  that  hatches  from  this  egg  creeps  along  this 
burrow  until  it  reaches  its  victim,  and  then  fastens  itself  to 
the  horn-tail  larva,  which  it  destroys  by  sucking  its  blood. 

The  larva  of  Thales- 
sa,  when  full  grown, 
changes  to  a  pupa 
within  the  burrow 
of  its  host,  and  the 
adult  gnaws  a  hole 
out  through  the  bark 
if  it  does  not  find  the 
hole  already  made  by 
the  Tremex." 

The  beetles  of 
the  family  Stylopidae 
present  an  interest- 
ing case  of  parasit- 
ism. The  adult  males  are  winged,  but  the  adult  females 
are  wingless  and  grub-like.  The  larval  stylopid  attaches 
itself  to  a  wasp  or  bee,  and  bores  into  its  abdomen.  It 
pupates  within  the  abdomen  of  the 
wasp  or  bee,  and  lies  there  with  its 
head  projecting  slightly  from  a  su- 
ture between  two  of  the  body  rings 
of  its  host  (Fig.  120).  The  adult 
finally  issues  and  leaves  the  host's 
body. 

Almost  all  of  the  mites  and  ticks, 
which  are  more  nearly  allied  to  the 
spiders  than  to  the  true  insects,  live 
parasitically.  Most  of  them  live  as 
external  parasites,  sucking  the  blood 
of  their  host,  but  some  live  under- 
neath the  skin  like  the  itch-mites 
(Fig.  121),  which  cause,  in  man,  the  disease  known  as 
the  itch. 


FIG.  121.— The  itch-mite 


PARASITISM  AND   DEGENERATION  193 

101.  Parasitic  vertebrates. — Among  the  vertebrate  ani- 
mals there  are  not  many  examples  of  true  parasitism.     The 
hag-fishes  or  borers  (Myxine,  Heptatrema,  Polistotrema)  are 
long  and  cylindrical,  eel-like  creatures,  very  slimy  and  very 
low  in  structure.     The  mouth  is  without  jaws,  but  forms  a 
sucking  disk,  by  which  the  hag-fish  attaches  itself  to  the 
body  of  some  other  fish.     By  means  of  the  rasping  teeth  on 
its  tongue,  it  makes  a  round  hole  through  the  skin,  usually 
at  the  throat.     It  then  devours  all  the  muscular  substance 
of  the  fish,  leaving  the  viscera  untouched.     When  the  fish 
finally  dies  it  is  a  mere  hulk  of  skin,  scales,  bones,  and 
viscera,  nearly  all  the  muscle  being  gone.     Then  the  hag- 
fish  slips  out  and  attacks  another  individual. 

The  lamprey,  another  low  fish,  in  similar  fashion  feeds 
leech-like  on  the  blood  of  other  fishes,  which  it  obtains  by 
lacerating  the  flesh  with  its  rasp-like  teeth,  remaining  at- 
tached by  the  round  sucking  disk  of  its  mouth. 

Certain  birds,  as  the  cow-bird  and  the  European  cuckoo, 
have  a  parasitic  habit,  laying  their  eggs  in  the  nests  of 
other  birds,  leaving  their  young  to  be  hatched  and  reared 
by  their  unwilling  hosts.  This  is,  however,  not  bodily  para- 
sitism, such  as  is  seen  among  lower  forms. 

102.  Degeneration  through  quiescence. — While  parasitism 
is  the  principal  cause  of  degeneration  among  animals,  yet 
it  is  not  the  sole  cause.     It  is  evident  that  if  for  any  other 
reason  animals  should  become  fixed,  and  live  inactive  or 
sedentary  lives,  they  would  degenerate.     And  there  are  not 
a  few  instances  of  degeneration  due  simply  to  a  quiescent 
life,  unaccompanied  by  parasitism.     The  Tunicata,  or  sea^ 
squirts  (Fig.  122),  are  animals  which  have  become  simple 
through  degeneration,  due  to  the  adoption  of  a  sedentary 
life,  the  withdrawal  from  the  crowd  of  animals  and  from 
the  struggle  which  it  necessitates.     The  young  tunicate  is 
a  free-swimming,  active,  tadpole-like  or  fish-like  creature, 
which  possesses  organs  very  like  those  of  the  adult  of  the 
simplest  fishes  or  fish-like  forms.     That  is,  the  sea-squirt 

14 


194 


ANIMAL  LIFE 


begins  life  as  a  primitively  simple  vertebrate.  It  possesses 
in  its  larval  stage  a  notochord,  the  delicate  structure  which 
precedes  the  formation  of  a  backbone,  extending  along  the 

upper  part  of  the  body, 
below  the  spinal  cord.  It 
is  found  in  all  young  ver- 
tebrates, and  is  charac- 
teristic of  the  class.  The 
other  organs  of  the  young 
tunicate  are  all  of  verte- 
bral type.  But  the  young 
sea-squirt  passes  a  period 
of  active  and  free  life  as 
a  little  fish,  after  which 
it  settles  down  and  at- 
taches itself  to  a  stone  or 
shell  or  wooden  pier  by 
means  of  suckers,  and  re- 
mains for  the  rest  of  its 
life  fixed.  Instead  of  go- 
ing on  and  developing 
into  a  fish-like  creature,  it 
loses  its  notochord,  its 
special  sense  organs,  and 

other  organs ;  it  loses  its  complexity  and  high  organiza- 
tion, and  becomes  a  "  mere  rooted  bag  with  a  double  neck." 
a  thoroughly  degenerate  animal. 

A  barnacle  is  another  example  of  degeneration  through 
quiescence.  The  barnacles  are  crustaceans  related  most 
nearly  to  the  crabs  and  shrimps.  The  young  barnacle  just 
from  the  egg  (Fig.  123,  /)  is  a  six-legged,  free-swimming 
nauplius,  very  like  a  young  prawn  or  crab,  with  single  eye. 
In  its  next  larval  stage  it  has  six  pairs  of  swimming  feet, 
two  compound  eyes,  and  two  large  antennae  or  feelers,  and 
still  lives  an  independent,  free-swimming  life.  When  it 
makes  its  final  change  to  the  adult  condition,  it  attaches 


PIG.  132.— A  sea-squirt,  or  tunicate. 


PARASITISM  AND  DEGENERATION 


195 


itself  to  some  stone  or  shell,  or  pile  or  ship's  bottom,  loses 
its  compound  eyes  and  feelers,  develops  a  protecting  shell, 
and  gives  up  all  power  of  locomotion.  Its  swimming  feet 
become  changed  into  grasping  organs,  and  it  loses  most  of 
its  outward  resemblances  to  the  other  members  of  its  class 
(Fig.  123,  e). 


FIG.  123.— Three  adult  crustaceans  and  their  larvae,  a,  prawn  (Peneus),  active  and 
free-living  ;  b,  larva  of  prawn  ;  c,  Sacculina,  parasite  ;  d,  larva  of  Sacculina ; 
e,  barnacle  (Lepas),  with  fixed  quiescent  life ;  /,  larva  of  barnacle.— After 
HAECKBL. 

Certain  insects  live  sedentary  or  fixed  lives.  All  the 
members  of  the  family  of  scale  insects  (Coccidae),  in  one 
sex  at  least,  show  degeneration,  that  has  been  caused  by 
quiescence.  One  of  these  coccids,  called  the  red  orange 
scale  (Fig.  124),  is  very  abundant  in  Florida  and  California 
and  in  other  orange-growing  regions.  The  male  is  a  beau- 
tiful, tiny,  two-winged  midge,  but  the  female  is  a  wingless, 


196 


ANIMAL  LIFE 


footless  little  sac  without  eyes  or  other  organs  of  special 
sense,  which  lies  motionless  under  a  flat,  thin,  circular,  red- 
dish scale  composed  of  wax  and  two  or  three  cast  skins  of 
the  insect  itself.  The  insect  has  a  long,  slender,  flexible, 
sucking  beak,  which  is  thrust  into  the  leaf  or  stem  or  fruit 
of  the  orange  on  which  the  "  scale  bug  "  lives  and  through 
which  the  insect  sucks  the  orange  sap,  which  is  its  only 


PIG.  124.-The  red  orange  scale  of  California,    a,  bit  of  leaf  with  scales  ;  b,  adult 
female ;  c,  wax  scale  under  which  adult  female  lives  ;  d,  larva ;  e,  adult  male. 

food.  It  lays  eggs  under  its  body,  and  thus  also  under  the 
protecting  wax  scale,  and  dies.  From  the  eggs  hatch  active 
little  larval  scale-bugs  with  eyes  and  feelers  and  six  legs. 
They  crawl  from  under  the  wax  scale  and  roam  about  over 
the  orange  tree.  Finally,  they  settle  down,  thrusting  their 
sucking  beak  into  the  plant  tissues,  and  cast  their  skin. 
The  females  lose  at  this  molt  their  legs  and  eyes  and 


PARASITISM  AND  DEGENERATION  197 

feelers.  Each  becomes  a  mere  motionless  sac  capable  only 
of  sucking  up  sap  and  of  laying  eggs.  The  young  males, 
however,  lose  their  sucking  beak  and  can  no  longer  take 
food,  but  they  gain  a  pair  of  wings  and  an  additional  pair 
of  eyes.  They  fly  about  and  fertilize  the  sac-like  females, 
which  then  molt  again  and  secrete  the  thin  wax  scale  over 
them. 

Throughout  the  animal  kingdom  loss  of  the  need  of 
movement  is  followed  by  the  loss  of  the  power  to  move,  and 
of  all  structures  related  to  it. 

103.  Degeneration  through  other  causes.— Loss  of  certain 
organs  may  occur  through  other  causes  than  parasitism  and 
a  fixed  life.  Many  insects  live  but  a  short  time  in  their 
adult  stage.  May-flies  live  for  but  a  few  hours  or,  at  most, 
a  few  days.  They  do  not  need  to  take  food  to  sustain  life 
for  so  short  a  time,  and  so  their  mouth  parts  have  become 
rudimentary  and  functionless  or  are  entirely  lost.  This  is 
true  of  some  moths  and  numerous  other  specially  short- 
lived insects.  Among  the  social  insects  the  workers  of  the 
termites  and  of  the  true  ants  are  wingless,  although  they 
are  born  of  winged  parents,  and  are  descendants  of  winged 
ancestors.  The  modification  of  structure  dependent  upon 
the  division  of  labor  among  the  individuals  of  the  com- 
munity has  taken  the  form,  in  the  case  of  the  workers,  of  a 
degeneration  in  the  loss  of  the  wings.  Insects  that  live 
in  caves  are  mostly  blind ;  they  have  lost  the  eyes,  whose 
function  could  not  be  exercised  in  the  darkness  of  the  cave. 
Certain  island-inhabiting  insects  have  lost  their  wings, 
flight  being  attended  with  too  much  danger.  The  strong 
sea-breezes  may  at  any  time  carry  a  flying  insect  oft3  the 
small  island  to  sea.  Only  those  which  do  not  fly  much  sur- 
vive, and  by  natural  selection  wingless  breeds  or  species  are 
produced.  Finally,  we  may  mention  the  great  modifications 
of  structure,  often  resulting  in  the  loss  of  certain  organs, 
which  take  place  to  produce  protective  resemblances  (see 
Chapter  XII).  In  such  cases  the  body  may  be  modified  in 


198  ANIMAL  LIFE 

color  and  shape  so  as  to  resemble  some  part  of  the  envi- 
ronment, and  thus  the  animal  may  be  unperceived  by  its 
enemies.  Many  insects  have  lost  their  wings  through  this 
cause. 

104.  Immediate  causes  of  degeneration. — When  we   say 
that  a  parasitic  or  quiescent  mode  of  life  leads  to  or  causes 
degeneration,  we  have  explained  the  stimulus  or  the  ulti- 
mate cause   of    degenerative   changes,  but   we  have  not 
shown  just  how  parasitism  or  quiescence  actually  produces 
these  changes.     Degeneration  or  the  atrophy  and  disap- 
pearance of  organs  or  parts  of  a  body  is  often  said  to  be 
due  to  disuse.     That  is,  the  disuse  of  a  part  is  believed  by 
many  naturalists  to  be  the  sufficient  cause  for  its  gradual 
dwindling  and  final  loss.     That  disuse  can  so  affect  parts 
of  a  body  during  the  lifetime  of  an  individual  is  true.     A 
muscle  unused  becomes  soft  and  flabby  and  small.    Whether 
the  effects  of  such  disuse  can  be  inherited,  however,  is  open 
to  serious  doubt.     Such  inheritance  must  be  assumed  if 
disuse  is  to  account  for  the  gradual  growing  less  and  final 
disappearance  of  an  organ  in  the  course  of  many  genera- 
tions.    Some  naturalists  believe  that  the  results  of  such 
disuse  can  be  inherited,  but  as  yet  such  belief  rests  on  no 
certain  knowledge.     If  characters  assumed  during  the  life- 
time of  the  individual  are  subject  to  inheritance,  disuse 
alone  may  explain  degeneration.    If  not,  some  other  imme- 
diate cause,  or  some  other  cause  along  with  disuse,  must 
be  found.    Such  a  cause  must  be  sought  for  in  the  action  of 
natural  selection,  preserving  the  advantages  of  simplicity  of 
structure  where  action  is  not  required. 

105.  Advantages  and  disadvantages  of  parasitism  and  de- 
generation.— We  are  accustomed,  perhaps,  to  think  of  degen- 
eration as  necessarily  implying  a  disadvantage  in  life.     A 
degenerate  animal  is  considered  to  be  not  the  equal  of  a  non- 
degenerate  animal,  and  this  would  be  true  if  both  kinds  of 
animals  had  to  face  the  same  conditions  of  life.     The  blind, 
footless,  simple,  degenerate  animal  could  not  cope  with  the 


PARASITISM  AND  DEGENERATION  199 

active,  keen-sighted,  highly  organized  non-degenerate  in 
free  competition.  But  free  competition  is  exactly  what 
the  degenerate  animal  has  nothing  to  do  with.  Certainly 
the  Sacculina  lives  successfully ;  it  is  well  adapted  for  its 
own  peculiar  kind  of  life.  For  the  life  of  a  scale  insect, 
no  better  type  of  structure  could  be  devised.  A  parasite 
enjoys  certain  obvious  advantages  in  life,  and  even  extreme 
degeneration  is  no  drawback,  but  rather  favors  it  in  the 
advantageousness  of  its  sheltered  and  easy  life.  As  long 
as  the  host  is  successful  in  eluding  its  enemies  and  avoid- 
ing accident  and  injury,  the  parasite  is  safe.  It  needs  to 
exercise  no  activity  or  vigilance  of  its  own ;  its  life  is  easy 
as  long  as  its  host  lives.  But  the  disadvantages  of  para- 
sitism and  degeneration  are  apparent  also.  The  fate  of  the 
parasite  is  usually  bound  up  with  the  fate  of  the  host. 
When  the  enemy  of  the  host  crab  prevails,  the  Sacculina 
goes  down  without  a  chance  to  struggle  in  its  own  defense. 
But  far  more  important  than  the  disadvantage  in  such  par- 
ticular or  individual  cases  is  the  disadvantage  of  the  fact 
that  the  parasite  can  not  adapt  itself  in  any  considerable 
degree  to  new  conditions.  It  has  become  so  specialized, 
so  greatly  modified  and  changed  to  adapt  itself  to  the  one 
set  of  conditions  under  which  it  now  lives ;  it  has  gone  so 
far  in  its  giving  up  of  organs  and  body  parts,  that  if  pres- 
ent conditions  should  change  and  new  ones  come  to  exist, 
the  parasite  could  not  adapt  itself  to  them.  The  independ- 
ent, active  animal  with  all  its  organs  and  all  its  functions 
intact,  holds  itself,  one  may  say,  ready  and  able  to  adapt 
itself  to  any  new  conditions  of  life  which  may  gradually 
come  into  existence.  The  parasite  has  risked  everything 
for  the  sake  of  a  sure  and  easy  life  under  the  presently 
existing  conditions.  Change  of  conditions  means  its  ex- 
tinction. 

106.  Human  degeneration. — It  is  not  proposed  in  these 
pages  to  discuss  the  application  of  the  laws  of  animal  life 
to  man.  But  each  and  every  one  extends  upward,  and  can 


200  ANIMAL  LIFE 

be  traced  in  the  relation  of  men  and  society.  Thus,  among 
men  as  among  animals,  self-dependence  favors  complexity 
of  power.  Dependence,  parasitism,  quiescence  favor  de- 
generation. Degeneration  means  loss  of  complexity,  the 
narrowing  of  the  range  of  powers  and  capabilities.  It  is 
not  necessarily  a  phase  of  disease  or  the  precursor  of  death. 
But  as  intellectual  and  moral  excellence  are  matters  associ- 
ated with  high  development  in  man,  dependence  is  unfa- 
vorable to  them. 

Degeneration  has  been  called  animal  pauperism.  Pau- 
perism in  all  its  forms,  whether  due  to  idleness,  pampering, 
or  misery,  is  human  degeneration.  It  has  been  shown  that 
a  large  part  of  the  criminality  and  pauperism  among  men 
is  hereditary,  due  to  the  survival  of  the  tendency  toward 
living  at  the  expense  of  others.  The  tendency  to  live  with- 
out self-activity  passes  from  generation  to  generation. 
Beggary  is  more  profitable  than  unskilled  and  inefficient 
labor,  and  our  ways  of  careless  charity  tend  to  propagate 
the  beggar.  That  form  of  charity  which  does  not  render 
its  recipient  self-helpful  is  an  incentive  toward  degenera- 
tion. Withdrawal  from  the  competition  of  life,  withdrawal 
from  self-helpful  activity,  aided  by  the  voluntary  or  invol- 
untary assistance  of  others — these  factors  bring  about  de- 
generation. The  same  results  follow  in  all  ages  and  with 
all  races,  with  the  lower  animals  as  with  men. 


CHAPTEK  XII 

PROTECTIVE   RESEMBLANCES,   AND    MIMICRY 

107.  Protective  resemblance  defined. — If  a  grasshopper 
be  startled  from  the  ground,  you  may  watch  it  and  deter- 
mine exactly  where  it  alights  after  its  leap  or  flight,  and 
yet,  on  going  to  the  spot,  be  wholly  unable  to  find  it.  The 
colors  and  marking  of  the  insect  so  harmonize  with  its  sur- 
roundings of  soil  and  vegetation  that  it  is  nearly  indistin- 
guishable as  long  as  it  remains  at  rest.  And  if  you  were 
intent  on  capturing  grasshoppers  for  fish-bait,  this  resem- 
blance in  appearance  to  their  surroundings  would  be  very 
annoying  to  you,  while  it  would  be  a  great  advantage  to 
the  grasshoppers,  protecting  some  of  them  from  capture  and 
death.  This  is  protective  resemblance.  Mere  casual  obser- 
vation reveals  to  us  that  such  instances  of  protective  resem- 
blance are  very  common  among  animals.  A  rabbit  or  grouse 
crouching  close  to  the  ground  and  remaining  motionless 
is  almost  indistinguishable.  Green  caterpillars  lying  out- 
stretched along  green  grass-blades  or  on  green  leaves  may 
be  touched  before  being  recognized  by  sight.  In  arctic 
regions  of  perpetual  snow  the  polar  bears,  the  snowy  arctic 
foxes,  and  the  hares  are  all  pure  white  instead  of  brown 
and  red  and  gray  like  their  cousins  of  temperate  and  warm 
regions.  Animals  of  the  desert  are  almost  without  excep- 
tion obscurely  mottled  with  gray  and  sand  color,  so  as  to 
harmonize  with  their  surroundings. 

In  the  struggle  for  existence  anything  that  may  give 
an  animal  an  advantage,  however  slight,  may  be  sufficient 
to  turn  the  scale  in  favor  of  the  organism  possessing  the 

201 


202  ANIMAL  LIFE 

advantage.  Such  an  advantage  may  be  swiftness  of  move- 
ment, or  unusual  strength  or  capacity  to  withstand  unfa- 
vorable meteorological  conditions,  or  the  possession  of  such 
color  and  markings  or  peculiar  shape  as  tend  to  conceal  the 
animal  from  its  enemies  or  from  its  prey.  Resemblances 
may  serve  the  purpose  of  aggression  as  well  as  protection. 
In  the  case  of  the  polar  bears  and  other  predaceous  ani- 
mals that  show  color  likenesses  to  their  surroundings,  the 
resemblance  can  better  be  called  aggressive  than  protective. 
The  concealment  afforded  by  the  resemblance  allows  them 
to  steal  unperceived  on  their  prey.  This,  of  course,  is  an 
advantage  to  them  as  truly  as  escape  from  enemies  would  be. 

We  have  already  seen  that  by  the  action  of  natural 
selection  and  heredity  those  variations  or  conditions  that 
give  animals  advantages  in  the  struggle  for  life  are  pre- 
served and  emphasized.  And  so  it  has  come  about  that 
advantageous  protective  resemblances  are  very  widespread 
among  animals,  and  assume  in  many  cases  extraordinarily 
striking  and  interesting  forms.  In  fact,  the  explanation 
of  much  of  the  coloring  and  patterning  of  animals  depends 
on  this  principle  of  protective  resemblance. 

Before  considering  further  the  general  conditions  of 
protective  resemblances,  it  will  be  advisable  to  refer  to 
specific  examples  classified  roughly  into  groups  or  special 
kinds  of  advantageous  colorings  and  markings. 

108.  General  protective  or  aggressive  resemblance. — As 
examples  of  general  protective  resemblance — that  is,  a  gen- 
eral color  effect  harmonizing  with  the  usual  surroundings 
and  tending  to  hide  or  render  indistinguishable  the  animal 
— may  be  mentioned  the  hue  of  the  green  parrots  of  the 
evergreen  tropical  forests ;  of  the  green  tree-frogs  and  tree- 
snakes  which  live  habitually  in  the  green  foliage ;  of  the 
mottled  gray  and  tawny  lizards,  birds,  and  small  mam- 
mals of  the  deserts ;  and  of  the  white  hares  and  foxes 
and  snowy  owls  and  ptarmigans  of  the  snow-covered  arc- 
tic regions.  Of  the  same  nature  is  the  slaty  blue  of  the 


204  ANIMAL  LIFE 

gulls  and  terns,  colored  like  the  sea.  In  the  brooks  most 
fishes  are  dark  olive  or  greenish  above  and  white  below. 
To  the  birds  and  other  enemies  which  look  down  on  them 
from  above  they  are  colored  like  the  bottom.  To  their  fish 
enemies  which  look  up  from  below,  their  color  is  like  the 
white  light  above  them,  and  their  forms  are  not  clearly 
seen.  The  fishes  of  the  deep  sea  in  perpetual  darkness  are 


FIG.  126.— Alligator  lizard  (Gerrhonottts  sdndcauda)  on  granite  rock.    Photograph 
by  J.  O.  SNYDEB,  Stanford  University,  California. 

inky  violet  in  color  below  as  well  as  above.  Those  that 
live  among  sea-weeds  are  red,  grass-green,  or  olive,  like 
the  plants  they  frequent.  General  protective  resemblance 
is  very  widespread  among  animals,  and  is  not  easily  appre- 
ciated when  the  animal  is  seen  in  museums  or  zoological 
gardens — that  is,  away  from  its  natural  or  normal  environ- 
ment. A  modification  of  general  color  resemblance  found 
in  many  animals  may  be  called  variable  protective  resem- 
blance. Certain  hares  and  other  animals  that  live  in 
northern  latitudes  are  wholly  white  during  the  winter  when 
the  snow  covers  everything,  but  in  summer,  when  much  of 
the  snow  melts,  revealing  the  brown  and  gray  rocks  and 


PROTECTIVE  RESEMBLANCES,  AND  MIMICRY      205 


withered  leaves,  these  creatures  change  color,  putting  on 
a  grayish  and  brownish  coat  of  hair.  The  ptarmigan  of 
the  Eocky  Mountains  (one  of  the  grouse),  which  lives  on 
the  snow  and  rocks  of  the  high  peaks,  is  almost  wholly 
white  in  winter,  but  in  summer  when  most  of  the  snow  is 
melted  its  plumage  is  chiefly  brown.  On  the  campus  at 
Stanford  University  there  is  a  little  pond  whose  shores  are 
covered  in  some  places  with  bits  of  bluish  rock,  in  other 
places  with  bits  of  reddish  rock,  and  in  still  other  places 
with  sand.  A  small  insect  called  the  toad-bug  ( Galgulus 
oculatus)  lives  abundantly  on  the  banks  of  this  pond. 
Specimens  collected  from  the  blue  rocks  are  bluish  in 
color,  those  from  the  red  rocks  are  reddish,  and  those  from 
the  sand  are  sand-colored.  Such  changes  of  color  to  suit 
the  changing  surroundings  can  be  quickly  made  in  the  case 
of  some  animals.  The  chameleons  of  the  tropics,  whose 
skin  changes  color  momentarily  from  green  to  brown, 
blackish  or  golden,  is  an  excellent  example  of  this  highly 
specialized  condition.  The  same  change  is  shown  by  a 
small  lizard  of  our  Southern  States  (AnoUus),  which  from  its 
habit  is  called  the  Florida 
chameleon.  There  is  a  lit- 
tle fish  (OUgocottus  snyderi) 
which  is  common  in  the  tide 
pools  of  the  bay  of  Monterey, 
in  California,  whose  color 
changes  quickly  to  harmo- 
nize with  the  different  colors 
of  the  rocks  it  happens  to 
rest  above.  Some  of  the  tree- 
frogs  show  this  variable  col- 
oring. A  very  striking  in- 
stance of  variable  protective 
resemblance  is  shown  by  the 

chrysalids  of  certain  butterflies.     An  eminent  English  nat- 
uralist collected  many  caterpillars  of  a  certain  species  of 


FIG.  l27.-Chryealid  of  swallow-tail  but- 
terfly (Papilio),  harmonizing  with  the 
bark  on  which  it  rests. 


206 


ANIMAL  LIFE 


butterfly,  and  put  them,  just  as  they  were  about  to  change 
into  pupae  or  chrysalids,  into  various  boxes,  lined  with  paper 
of  different  colors.  The  color  of  the  chrysalid  was  found 


FIG.  128. — Chrysalld  of  butterfly  (lower  left-hand  projection  from  stem),  showing  pro- 
tective resemblance.    Photograph  from  Nature. 

to  harmonize  very  plainly  with  the  color  of  the  lining  of 
the  box  in  which  the  chrysalid  hung.  It  is  a  familiar  fact 
to  entomologists  that  most  butterfly  chrysalids  resemble  in 


PROTECTIVE  RESEMBLANCES,  AND  MIMICRY     207 

color  and  general  external  appearance  the  surface  of  the 
object  on  which  they  rest  (Figs.  127  and  128). 

109.  Special  protective  resemblance.— Far  more  striking 
are  those  cases  of  protective  resemblance  in  which  the  ani- 
mal resembles  in  color  and  shape,  sometimes  in  extraor- 
dinary detail,  some  particular  object  or  part  of  its  usual 
environment.  Certain  parts  of  the  Atlantic  Ocean  are 
covered  with  great  patches  of  sea-weed  called  the  gulf-weed 
(Sargassum),  and  many  kinds  of  animals — fishes  and  other 
creatures — live  upon  and  among  the  algae.  Xo  one  can 
fail  to  note  the  extraordinary  color  resemblances  which  exist 
between  those  animals  and  the  weed  itself.  The  gulf-weed 
is  of  an  olive-yellow  color,  and  the  crabs  and  shrimps,  a  cer- 
tain flat-worm,  a  certain  mollusk,  and  a  little  fish,  all  of 
which  live  among  the  Sargatmm,  are  exactly  of  the  same 
shade  of  yellow  as  the  weed,  and  have  small  white  markings 
on  their  bodies  which  are  characteristic  also  of  the  tiargas- 
fsurn.  The  mouse-fish  or  Sargassum  fish  and  the  little  sea- 
horses, often  attached  to  the  gulf  weed,  show  the  same  traits 
of  coloration  (Fig.  129).  In  the  black  rocks  about  Tahiti 
is  found  the  black  nokee  or  lava-fish  (Emmydrichthys  vul- 
canus)  (Fig.  66),  which  corresponds  perfectly  in  color  and 
form  to  a  piece  of  lava.  This  fish  is  also  noteworthy  for 
having  envenomed  spines  in  the  fin  on  its  back.  The 
slender  grass-green  caterpillars  of  many  moths  and  butter- 
flies resemble  very  closely  the  thin  grass-blades  among 
which  they  live.  The  larvae  of  the  geometrid  moths,  called 
inch-worms  or  span-worms,  are  twig-like  in  appearance, 
and  have  the  habit,  when  disturbed,  of  standing  out  stiffly 
from  the  twig  or  branch  upon  which  they  rest,  so  as  to  re- 
semble in  position  as  well  as  in  color  and  markings  a  short 
or  a  broken  twig.  One  of  the  most  striking  resemblances 
of  this  sort  is  shown  by  the  large  geometrid  larva  illus- 
trated in  Fig.  130,  which  was  found  near  Ithaca,  New  York. 
The  body  of  this  caterpillar  has  a  few  small,  irregular  spots 
or  humps,  resembling  very  exactly  the  scars  left  by  fallen 


Fia.  129.— The  mouse-fish  (Pterophryne  histrio)  in  the  Sargassum  or  gulf-weed.  The 
fishes  are  marked  and  colored  so  as  to  be  nearly  indistinguishable  from  the  masses 
of  the  gulf- weed.  In  the  lower  right-hand  corner  of  figure  are  two  sea-horses,  also 
shaped  and  marked  so  as  to  be  concealed. 


PROTECTIVE  RESEMBLANCES,  AND  MIMICRY      209 

buds  or  twigs.     These  caterpillars  have  a  special  muscular 
development  to  enable  them  to  hold  themselves  rigidly  for 


PIG.  130. — A  geometrid  larva  on  a  branch.   (The       FIG.  131. — A  walking-stick  insect 
larva  is  the  upper  right-hand  projection  from  (Diapheromera  femorata)   on 

the  stem.)  twig. 

long  times  in  this  trying  attitude.     They  also  lack  the 
middle  prop-legs  of  the  body,  common  to  other  lepidopter- 
15 


210 


ANIMAL  LIFE 


ous  larvae,  the  presence  of  which  would  tend  to  destroy  the 
illusion  so  successfully  carried  out  by  them.  The  common 
walking-stick  (Diapheromera)  (Fig.  131),  with  its  wingless, 
greatly  elongate,  dull-colored  body,  is  an  excellent  example 
of  special  protective  resemblance.  It  is  quite  indistinguish- 
able, when  at  rest,  from  the  twigs  to  which  it  is  clinging. 
Another  member  of  the  family  of  insects  to  which  the  walk- 
ing-stick belongs  is  the  famous  green-leaf  insect  (Phy Ilium) 

(Fig.  132).  It  is  found  in 
South  America  and  is  of  a 
bright  green  color,  with  broad 
leaf -like  wings  and  body,  with 
markings  which  imitate  the 
leaf  veins,  and  small  irregu- 
lar yellowish  spots  which 
mimic  decaying  or  stained 
or  fungus-covered  spots  in 
the  leaf. 

There  are  many  butter- 
flies that  resemble  dead 
leaves.  All  our  common 
meadow  browns  ( Grapta), 
brown  and  reddish  butter- 
flies with  ragged-edged  wings, 
that  appear  in  the  autumn 


\ 


FIG.  132.— The  green-leaf  insect 
(Phyttium). 


and  flutter  aimlessly  about  ex- 
actly like  the  falling  leaves, 
show  this  resemblance.  But 
most  remarkable  of  all  is  a 

large  butterfly  (Kallima)  (Fig.  133)  of  the  East  Indian 
region.  The  upper  sides  of  the  wings  are  dark,  with 
purplish  and  orange  markings,  not  at  all  resembling  a 
dead  leaf.  But  the  butterflies  when  at  rest  hold  their 
wings  together  over  the  back,  so  that  only  the  under  sides 
of  the  wings  are  exposed.  The  under  sides  of  Kallima's 
wings  are  exactly  the  color  of  a  dead  and  dried  leaf,  and 


PROTECTIVE  RESEMBLANCES,  AND  MIMICRY      211 

the  wings  are  so  held  that  all  combine  to  mimic  with  ex- 
traordinary fidelity  a  dead  leaf  still  attached  to  the  twig  by 
a  short  pedicle  or  leafstalk  imitated  by  a  short  tail  on  the 


FIG.  133.—Xallima,  the  "  dead-leaf  butterfly." 

hind  wings,  and  showing  midrib,  oblique  veins,  and,  most 
remarkable  of  all,  two  apparent  holes,  like  those  made  in 
leaves  by  insects,  but  in  the  butterfly  imitated  by  two  small 
circular  spots  free  from  scales  and  hence  clear  and  trans- 


212  ANIMAL  LIFE 

parent.     With  the  head  and  feelers  concealed  beneath  the 
wings,  it  makes  the  resemblance  wonderfully  exact. 

There   are  numerous  instances   of    special    protective 
resemblance  among  spiders.     Many  spiders  (Fig.  134)  that 


FIG.  134.— Spiders  showing  unusual  shapes  and  patterns,  for  purposes  of 
aggressive  resemblance. 

live  habitually  on  tree  trunks  resemble  bits  of  bark  or  small, 
irregular  masses  of  lichen.  A  whole  family  of  spiders, 
which  live  in  flower-cups  lying  in  wait  for  insects,  are  white 
and  pink  and  party-colored,  resembling  the  markings  of  the 
special  flowers  frequented  by  them.  This  is,  of  course,  a 


FIG.  135.— A  pipe-fish  (Phyllopteryx)  resembling  sea-weed,  in  which  it  lives 


special  resemblance  not  so  much  for  protection  as  for  ag- 
gression ;  the  insects  coming  to  visit  the  flowers  are  unable 
to  distinguish  the  spiders  and  fall  an  easy  prey  to  them. 

110.  Warning  colors  and  terrifying  appearances. — In  the 
cases  of  advantageous  coloring  and  patterning  so  far  dis- 


PROTECTIVE  RESEMBLANCES,  AND  MIMICRY     213 

cussed  the  advantage  to  the  animal  lies  in  the  resemblance 
between  the  animals  and  their  surroundings,  in  the  incon- 
spicuousness  and  concealment  afforded  by  the  coloration. 
But  there  is  another  interesting  phase  of  advantageous 
coloration  in  which  the  advantage  derived  is  in  render- 
ing the  animals  as  conspicuous  and  as  readily  recogniz- 
able as  possible.  While  many  animals  are  very  inconspicu- 
ously colored,  or  are  manifestly  colored  so  as  to  resemble 
their  surroundings,  generally  or  specifically,  many  other 
animals  are  very  brightly  and  conspicuously  colored  and 
patterned.  If  we  are  struck  by  the  numerous  cases  of  imi- 
tative coloring  among  insects,  we  must  be  no  less  impressed 
by  the  many  cases  of  bizarre  and  conspicuous  coloration 
among  them. 

Many  animals,  as  we  well  know,  possess  special  and 
effective  weapons  of  defense,  as  the  poison-fangs  of  the 
venomous  snakes  and  the  stings  of  bees  and  wasps.  Other 
animals,  and  with  these  cases  most  of  us  are  not  so  well 
acquainted,  possess  a  means  of  defense,  or  rather  safety,  in 
being  inedible — that  is,  in  possessing  some  acrid  or  ill- 
tasting  substance  in  the  body  which  renders  them  unpala- 
table to  predaceous  animals.  Many  caterpillars  have  been 
found,  by  observation  in  Nature  and  by  experiment,  to  be 
distasteful  to  insectivorous  birds.  Now,  it  is  obvious  that 
it  would  be  a  great  advantage  to  these  caterpillars  if  they 
could  be  readily  recognized  by  birds,  for  a  severe  stroke  by 
a  bird's  bill  is  about  as  fatal  to  a  caterpillar  as  being  wholly 
eaten.  Its  soft,  distended  body  suffers  mortal  hurt  if  cut 
or  bitten  by  the  bird's  beak.  This  advantage  of  being 
readily  recognizable  is  possessed  by  many  if  not  all  ill- 
tasting  caterpillars  by  being  brilliantly  and  conspicuously 
colored  and  marked.  Such  colors  and  markings  are  called 
warning  colors.  They  are  intended  to  inform  birds  of  the 
fact  that  the  caterpillar  displaying  them  is  an  ill-tasting 
insect,  a  caterpillar  to  be  let  alone.  The  conspicuously 
black-and-yellow  banded  larva  (Fig.  43,  b)  of  the  common 


214  ANIMAL  LIFE 

Monarch  butterfly  is  a  good  example  of  the  possession  of 
warning  colors  by  distasteful  caterpillars. 

These  warning  colors  are  possessed  not  only  by  the  ill- 
tasting  caterpillars,  but  by  many  animals  which  have  spe- 
cial means  of  defense.  The  wasps  and  bees,  provided  with 
stings — dangerous  animals  to  trouble — are  almost  all  con- 
spicuously marked  with  yellow  and  black.  The  lady-bird 
beetles  (Fig.  136),  composing  a  whole  family  of  small  beetles 


Fro.  136.— Two  lady-bird  beetles,  conspicuously  colored  and  marked. 

which  are  all  ill-tasting,  are  brightly  and  conspicuously  col- 
ored and  spotted.  The  Gila  monster  (ffeloderma),ihe  only 
poisonous  lizard,  differs  from  most  other  lizards  in  being 
strikingly  patterned  with  black  and  brown.  Some  of  the 
venomous  snakes  are  conspicuously  colored,  as  the  coral 
snakes  (Elaps)  or  coralillos  of  the  tropics.  The  naturalist 
Belt,  whose  observations  in  Nicaragua  have  added  much  to 
our  knowledge  of  tropical  animals,  describes  as  follows  an 
interesting  example  of  warning  colors  in  a  species  of  frog : 
'•'In  the  woods  around  Santo  Domingo  (Nicaragua)  there 
are  many  frogs.  Some  are  green  or  brown  and  imitate 
green  or  dead  leaves,  and  live  among  foliage.  Others  are 
dull  earth-colored,  and  hide  in  holes  or  under  logs.  All 
these  come  out  only  at  night  to  feed,  and  they  are  all 
preyed  upon  by  snakes  and  birds.  In  contrast  with  these 
obscurely  colored  species,  another  little  frog  hops  about  in 


PROTECTIVE  RESEMBLANCES,  AND  MIMICRY      215 

the  daytime,  dressed  in  a  bright  livery  of  red  and  blue. 
He  can  not  be  mistaken  for  any  other,  and  his  flaming 
breast  and  blue  stockings  show  that  he  does  not  court  con- 
cealment. He  is  very  abundant  in  the  damp  woods,  and  I 
was  convinced  he  was  uneatable  so  soon  as  I  made  his 
acquaintance  and  saw  the  happy  sense  of  security  with 
which  he  hopped  about.  I  took  a  few  specimens  home 
with  me,  and  tried  my  fowls  and  ducks  with  them,  but 
none  would  touch  them.  At  last,  by  throwing  down  pieces 
of  meat,  for  which  there  was  a  great  competition  among 
them,  I  managed  to  entice  a  young  duck  into  snatching  up 
one  of  the  little  frogs.  Instead  of  swallowing  it,  however, 
it  instantly  threw  it  out  of  its  mouth,  and  went  about  jerk- 
ing its  head,  as  if  trying  to  throw  off  come  unpleasant 
taste." 

Certain  animals  which  are  without  special  means  of 
defense  and  are  not  at  all  formidable  or  dangerous  are  yet 
so  marked  or  shaped  and  so  behave  as  to  present  a  threat- 
ening or  terrifying  appearance.  The  large  green  caterpil- 
lars (Fig.  137)  of  the  Sphinx  moths — the  tomato-worm  is  a 
familiar  one  of  these  larvae — have  a  formidable-looking, 


PIG.  137.— A  "tomato-worm"  larva  of  the  Sphinx  moth,  Pldegethontius  Carolina, 
showing  terrifying  appearance. 

sharp  horn  on  the  back  of  the  next  to  last  body  ring. 
When  disturbed  they  lift  the  hinder  part  of  the  body,  bear- 
ing the  horn,  and  move  it  about  threateningly.  As  a  mat- 
ter of  fact,  the  horn  is  not  at  all  a  weapon  of  defense,  but  is 
quite  harmless.  Numerous  insects  when  disturbed  lift 
the  hind  part  of  the  body,  and  by  making  threatening  mo- 


216 


ANIMAL  LIFE 


tions  lead  enemies  to  believe  that  they  possess  a  sting. 
The  striking  eye-spots  of  many  insects  are  believed  by  some 
entomologists  to  be  of  the  nature  of  terrifying  appearances. 
The  larva  (Fig.  138)  of  the  Puss  moth  (Cerura)  has  been 
often  referred  to  as  a  striking  example  of  terrifying  appear- 
ances. When  one  of  these  larvae  is  disturbed,  "  it  retracts 

its  head  into  the 
first  body  ring  in- 
flating the  mar- 
gin, which  is  of  a 
bright  red  color. 
There  are  two  in- 
tensely black  spots 
on  this  margin  in  the 
appropriate  position  for 
eyes,  and  the  whole  ap- 
pearance is  that  of  a  large 
flat  face  extending  to  the 
outer  edge  of  the  red  mar- 
gin. The  effect  is  an  in- 
tensely exaggerated  cari- 
cature of  a  vertebrate 
face,  which  is  probably 
alarming  to  the  verte- 
brate enemies  of  the  cat- 
erpillar. .  .  .  The  effect  is  also  greatly  strengthened  by  two 
pink  whips  which  are  swiftly  protruded  from  the  prongs 
of  the  fork  in  which  the  body  terminates.  .  .  .  The  end 
of  the  body  is  at  the  same  time  curved  forward  over  the 
back,  so  that  the  pink  filaments  are  brandished  above  the 
head." 

111.  Alluring  coloration. — A  few  animals  show  what  are 
called  alluring  colors — that  is,  they  display  a  color  pattern 
so  arranged  as  to  resemble  or  mimic  a  flower  or  other  lure, 
and  thus  to  entice  to  them  other  animals,  their  natural  prey. 
This  is  a  special  kind  of  aggressive  resemblance.  A  species 


PIG.  138.— Larva  of  the  Puss  moth  (Cerura). 
Upper  figure  shows  the  larva  as  it  appears 
when  undisturbed  ;  lower  figure,  when  dis- 
turbed.— After  POULTON. 


PROTECTIVE  RESEMBLANCES,  AND  MIMICRY     217 

of  predatory  insect  called  a  "  praying-horse  "  (allied  to  the 
genus  Mantis),  found  in  India,  has  the  shape  and  color  of 
an  orchid.  Small  insects  are  attracted  and  fall  a  prey  to  it. 
Certain  Brazilian  fly-catching  birds  have  a  brilliantly  colored 
crest  which  can  be  displayed  in  the  shape  of  a  flower-cup. 
The  insects  attracted  by  the  apparent  flower  furnish  the  fly- 
catcher with  food.  An  Asiatic  lizard  is  wholly  colored  like 
the  sand  upon  which  it  lives  except  for  a  peculiar  red  fold 
of  skin  at  each  angle  of  the  mouth.  This  fold  is  arranged 
in  flower-like  shape,  "  exactly  resembling  a  little  red  flower 
which  grows  in  the  sand."  Insects  attracted  by  these 
flowers  find  out  their  mistake  too  late.  In  the  tribe  of 
fishes  called  the  "  anglers  "  or  fishing  frogs  the  front  rays 
of  the  dorsal  fin  are  prolonged  in  shape  of  long,  slender  fila- 
ments, the  foremost  and  longest  of  which  has  a  flattened 
and  divided  extremity  like  the  bait  on  a  hook.  The  fish 
conceals  itself  in  the  mud  or  in  the  cavities  of  a  coral  reef 
and  waves  the  filaments  back  and  forth.  Small  fish  are  at- 
tracted by  the  lure,  mistaking  it  for  worms  writhing  about 
in  the  water  or  among  the  weeds.  As  they  approach  they 
are  ingulfed  in  the  mouth  of  the  angler,  which  in  some  of 
the  species  is  of  enormous  size.  One  of  these  species  is 
known  to  fishermen  as  the  "all-mouth."  These  fishes 
(Lopliius  piscatorius),  which  live  in  the  mud,  are  colored 
like  mud  or  clay.  Other  forms  of  anglers,  living  among 
coral  reefs,  are  brown  and  red  (Antennarius),  their  colora- 
tion imitating  in  minutest  detail  the  markings  and  out- 
growths on  the  reef  itself,  the  lure  itself  imitating  a  worm 
of  the  reef.  In  a  certain  group  of  deep-sea  anglers,  the  sea- 
devils  ( Ceratiidce),  certain  species  show  a  still  further  spe- 
cialization of  the  curious  fishing-rod.  In  one  species  ((70- 
rynolophus  reinhardti)  (Fig.  54),  living  off  the  coast  of 
Greenland  at  a  depth  of  upward  of  a  mile,  the  fishing-rod 
or  first  dorsal  spine  has  a  luminous  bulb  at  its  tip  around 
which  are  fleshy,  worm-like  streamers.  At  the  abyssal 
depths  of  a  mile,  more  or  less,  frequented  by  these  sea- 


218  ANIMAL  LIFE 

devils  thex'e  is  no  light,  the  inky  darkness  being  absolute. 
This  shining  lure  is  therefore  a  most  effective  means  of 
securing  food. 

112.  Mimicry. — Although  the  word  mimicry  could  often 
have  been  used  aptly  in  the  foregoing  account  of  protective 
resemblances,  it  has  been  reserved  for  use  in  connection 
with  a  certain  specific  group  of  cases.  It  has  been  reserved 
to  be  applied  exclusively  to  those  rather  numerous  instances 
where  an  otherwise  defenseless  animal,  one  without  poison- 
fangs  or  sting,  and  without  an  ill-tasting  substance  in  its 
body,  mimics  some  other  specially  defended  or  inedible  ani- 
mal sufficiently  to  be  mistaken  for  it  and  so  to  escape 
attack.  Such  cases  of  protective  resemblance  are  called 
true  mimicry,  and  they  are  especially  to  be  observed  among 
insects. 

In  Fig.  139  are  pictured  three  familiar  American  butter- 
flies. One  of  these,  the  Monarch  butterfly  (Anosia  plexip- 
pus),  is  perhaps  the  most  abundant  and  widespread  butter- 
fly of  our  country.  It  is  a  fact  well  known  to  entomologists 
that  the  Monarch  is  distasteful  to  birds  and  is  let  alone  by 
them.  It  is  a  conspicuous  butterfly,  being  large  and  chiefly 
of  a  red-brown  color.  The  Viceroy  butterfly  (Basilarchia 
archippus),  also  red-brown  and  much  like  the  Monarch,  is 
not,  as  its  appearance  would  seem  to  indicate,  a  very  near 
relative  of  the  Monarch,  belonging  to  the  same  genus,  but 
on  the  contrary  it  belongs  to  the  same  genus  with  the  third 
butterfly  figured,  the  black  and  white  BasilarcMa.  All  the 
butterflies  of  the  genus  Basilarchia  are  black  and  white 
except  this  species,  the  Viceroy,  and  one  other.  The  Vice- 
roy is  not  distasteful  to  birds ;  it  is  edible,  but  it  mimics  the 
inedible  Monarch  so  closely  that  the  deception  is  not  de- 
tected by  the  birds,  and  so  it  is  not  molested. 

In  the  tropics  there  have  been  discovered  numerous 
similar  instances  of  mimicry  by  edible  butterflies  of  inedi- 
ble kinds.  The  members  of  two  great  families  of  butterflies 
(Danaidae  and  Heliconidae)  are  distasteful  to  birds,  and  are 


FIG.  139.— The  mimicking  of  the  inedible  Monarch  butterfly  by  the  edible  Viceroy. 
Upper  figure  is  the  Monarch  (Anosia  plexippus} ;  middle  figure  is  the  Viceroy 
(Basilarchia  archippus) ;  lowest  figure  is  another  member  of  the  same  genus 
(Basilarchia),  to  show  the  usual  color  pattern  of  the  species  of  the  genus. 


220 


ANIMAL  LIFE 


mimicked  by  members  of  the  other  butterfly  families  (espe- 
cially the  Pieridae),  to  which  family  our  common  white 
cabbage-butterfly  belongs,  and  by  the  swallow-tails  (Papi- 
lionidae). 

The  bees  and  wasps  are  protected  by  their  stings.  They 
are  usually  conspicuous,  being  banded  with  yellow  and  black. 
They  are  mimicked  by  numerous  other  insects,  especially 
moths  and  flies,  two  defenseless  kinds  of  insects.  This 
mimicking  of  bees  and  wasps  by  flies  is  very  common,  and 
can  be  observed  readily  at  any  flowering  shrub.  The  flower- 
flies  (Syrphidae),  which,  with  the  bees,  visit  flowers,  can  be 
distinguished  from  the  bees  only  by  sharp  observing.  When 
these  bees  and  flies  can  be  caught  and  examined  in  hand,  it 
will  be  found  that  the  flies  have  but  two  wings  while  the 
bees  have  four. 

A  remarkable  and  interesting  case  of  mimicry  among 
insects  of  different  orders  is  that  of  certain  South  Ameri- 
can tree-hoppers  (of  the  family  Membracidae,  of  the  order 
Hemiptera),  which  mimic  the  famous  leaf -cutting  ant 
(Saula)  of  the  Amazons  (Fig.  140).  These  ants  have  the 
curious  habit  of  cutting  off,  with  their  sharp  jaws,  bits  of 

green  leaves  and  carry- 
ing them  to  their  nests. 
In  carrying  the  bits  of 
leaves  the  ants  hold  them 
vertically  above  their 
heads.  The  leaf-hoppers 
mimic  the  ants  and  their 
burdens  with  remarka- 
ble exactitude  by  having 
the  back  of  the  body  ele- 
vated in  the  form  of  a 
thin,  jagged-edged  ridge  no  thicker  than  a  leaf.  This  part 
of  the  body  is  green  like  the  leaves,  while  the  under  part 
of  the  body  and  the  legs  are  brown  like  the  ants. 

Some  examples  of  mimicry  among  other  animals  than 


FIG.  140.— Tree-hopper  (Membracidae),  which 
mimics  the  leaf-cutting  ant  (Sauba)  of  Bra- 
zil. (Upper  right-hand  insect  is  the  tree- 
hopper.) 


PROTECTIVE  RESEMBLANCES,  AND  MIMICRY     221 

insects  are  known,  but  not  many.  The  conspicuously 
marked  venomous  coral-snake  or  coralillos  (Elaps)  is  mim- 
icked by  certain  non-venomous  snakes  called  king-snakes 
(Lampropeltis,  Osceola).  The  pattern  of  red  and  black 
bands  surrounding  the  cylindrical  body  is  perfectly  imi- 
tated. But  whether  this  is  true  mimicry  brought  about 
for  purposes  of  protection  may  be  doubted.  Instances 
among  birds  have  been  described,  and  a  single  case  has 
been  recorded  in  the  class  of  mammals.  But  it  is  among 
the  insects  that  the  best  attested  instances  occur.  The 
simple  fact  of  the  close  resemblance  of  two  widely  related 
animals  can  not  be  taken  to  prove  the  existence  of  mimicry. 
Two  animals  may  both  come  to  resemble  some  particular 
part  in  their  common  environment  and  thus  to  resemble 
closely  each  other.  Here  we  have  simply  two  instances 
of  special  protective  resemblance,  and  not  an  instance  of 
mimicry.  The  student  of  zoology  will  do  well  to  watch 
sharply  for  examples  of  protective  resemblance  or  mimicry, 
for  but  few  of  the  instances  that  undoubtedly  exist  are  as 
yet  known. 

113.  Protective  resemblances  and  mimicry  most  common 
among  insects, — The  large  majority  of  the  preceding  exam- 
ples have  been  taken  from  among  the  insects.  This  is 
explained  by  the  fact  that  the  phenomena  of  protective 
resemblances  and  mimicry  have  been  studied  especially 
among  insects;  the  theory  of  mimicry  was  worked  out 
chiefly  from  the  observation  and  study  of  the  colors  and 
markings  of  insects  and  of  the  economy  of  insect  life. 
Why  protective  resemblances  and  mimicry  among  insects 
have  been  chiefly  studied  is  because  these  conditions  are 
specially  common  among  insects.  The  great  class  Insecta 
includes  more  than  two  thirds  of  all  the  known  living 
species  of  animals.  The  struggle  for  existence  among  the 
insects  is  especially  severe  and  bitter.  All  kinds  of  "  shifts 
for  a  living  "  are  pushed  to  extremes ;  and  as  insect  colors 
and  patterns  are  especially  varied  and  conspicuous,  it  is 


222  ANIMAL  LIFE 

only  to  be  expected  that  this  useful  modification  of  colors 
and  patterns,  that  results  in  the  striking  phenomena  of 
special  protective  resemblances  and  mimicry,  should  be 
specially  widespread  and  pronounced  among  insects.  More- 
over, they  are  mostly  deficient  in  other  means  of  defense, 
and  seem  to  be  the  favorite  food  for  many  different  kinds 
of  animals.  Protective  resemblance  is  their  best  and  most 
widely  adopted  means  of  preserving  life. 

114.  No  volition  in  mimicry. — The  use  of  the  word  mim- 
icry has  been  criticised  because  it  suggests  the  exercise  of 
volition  or  intent  on  the  part  of  the  mimicking  animal. 
The  student  should  not  entertain  this  conception  of  mim- 
icry.    In  the  use  of   "mimicry"  in  connection  with  the 
phenomena  just   described,  the  biologist  ascribes  to  it  a 
technical  meaning,  which  excludes  any  suggestion  of  voli- 
tion or  intent  on  the  part  of  the  mimic.     Just  how  such 
extraordinary  and  perfect  cases  of  mimicry  as  shown  by 
Phyllium  and  Kallima  have  come  to  exist  is  a  problem 
whose  solution  is  not  agreed  on  by  naturalists,  but  none  of 
them  makes  volition — the  will  or  intent  of  the  animal — any 
part  of  his  proposed  solution.     Each  case  of  mimicry  is  the 
result  of  a  slow  and  gradual  change,  through  a  long  series 
of  ancestors.     The  mimicry  may  indeed  include  the  adop- 
tion of  certain  habits  of  action  which  strengthen  and  make 
more  pronounced  the  deception  of  shape  and  color.     But 
these  habits,  too,  are  the  result  of  a  long  development,  and 
are  instinctive  or  reflex — that  is,  performed  without  the 
exercise  of  volition  or  reason. 

115.  Color;  its  utility  and  beauty. — The  causes  of  color, 
and  the  uses  of  color  in  animals  and  in  plants  are  subjects 
to  which  naturalists  have  paid  and  are  paying  much  atten- 
tion.    The  subject  of  "  protective  resemblances  and  mim- 
icry" is   only  one,  though   one   of  the  most   interesting, 
branches  or  subordinate  subjects  of  the  general  theory  of 
the  uses  of  color.     Other  uses  are  obvious.     Bright  colors 
and  markings  may  serve  for  the  attraction  of  mates ;  thus 


PROTECTIVE  RESEMBLANCES,  AND  MIMICRY      223 

are  explained  by  some  naturalists  the  brilliant  plumage  of 
the  male  birds,  as  in  the  case  of  the  bird-of-paradise  and 
the  pheasants.  Or  they  may  serve  for  recognition  charac- 
ters, enabling  the  individuals  of  a  band  of  animals  readily 
to  recognize  their  companions ;  the  conspicuous  whiteness 
of  the  short  tail  of  the  antelopes  and  cotton-tail  rabbits, 
the  black  tail  of  the  black-tail  deer,  and  the  white  tail- 
feathers  of  the  meadow-lark,  are  explained  by  many  natu- 
ralists on  this  ground.  Eecognition  marks  of  this  type 
are  especially  numerous  among  the  birds,  hardly  a  species 
being  without  one  or  more  of  them,  if  their  meaning  is  cor- 
rectly interpreted.  The  white  color  of  arctic  animals  may 
be  useful  not  alone  in  rendering  them  inconspicuous,  but 
may  serve  also  a  direct  physiological  function  in  preventing 
the  loss  of  heat  from  the  body  by  radiation.  And  the  dark 
colors  of  animals  may  be  of  value  to  them  in  absorbing  heat 
rays  and  thus  helping  them  to  keep  warm.  But  "  by  far 
the  most  widespread  use  of  color  is  to  assist  an  animal  in 
escaping  from  its  enemies  or  in  capturing  its  prey." 

The  colors  of  an  animal  may  indeed  not  be  useful  to 
it  at  all.  Many  color  patterns  exist  on  present-day  birds 
simply  because,  preserved  by  heredity,  they  are  handed 
down  by  their  ancestors,  to  whom,  under  different  condi- 
tions of  life,  they  may  have  been  of  direct  use.  For  the 
most  part,  however,  we  can  look  on  the  varied  colors  and 
the  striking  patterns  exhibited  by  animals  as  being  in  some 
way  or  another  of  real  use  and  value.  We  can  enjoy  the 
exquisite  coloration  of  the  wings  of  a  butterfly  none  the 
less,  however,  because  we  know  that  these  beautiful  colors 
and  their  arrangement  tend  to  preserve  the  life  of  the 
dainty  creature,  and  have  been  produced  by  the  operation 
of  fixed  laws  of  Nature  working  through  the  ages. 


CHAPTEE  XIII 

THE   SPECIAL   SENSES 

116.  Importance  of   the  special  senses. — The  means  by 
which  animals  become  acquainted  with  the  outer  world 
are  the   special  senses,  such  as  feeling,  tasting,  smelling, 
hearing,  and  seeing.     The  behavior  of  animals  with  regard 
to  their  surroundings,  with  regard  to  all  the  world  outside 
of  their  own  body,  depends  upon  what  they  learn  of  this 
outer  world  through  the  exercise  of  these  special  senses. 
Habits  are  formed  on  the  basis  of  experience  or  knowledge 
of  the  outer  world  gained  by  the  special  senses,  and  the 
development  of  the  power  to  reason  or  to  have  sense  de- 
pends on  their  pre-existence. 

117.  Difficulty  of  the  study  of  the  special  senses. — We  are 
accustomed  to  think  of  the  organs  of  the  special  senses  as 
extremely  complex  parts  of  the  body,  and  this  is  certainly 
true  in  the  case  of  the  higher  animals.     In  our  own  body, 
the  ears  and  eyes  are  organs  of  most  specialized  and  highly 
developed  condition.     But  we  must  not  overlook  the  fact 
that  the  animal  kingdom  is  composed  of  creatures  of  widely 
varying  degrees  of  organization,  and  that  in  any  considera- 
tion of  matters  common  to  all  animals  those  animals  of 
simplest  and  most  lowly  organization  must  be  studied  as 
well  as  those  of  high  development.     The  study  of  the  spe- 
cial senses  presents  two  phases,  namely,  the  study  of  the 
structure  of  the  organs  of  special  sense,  and  the  study  of 
the  physiology  of  special  sense — that  is,  the  functions  of 
these  organs.     It  will  be  recognized  that  in  the  study  of 
how  other  animals  feel  and  taste  and  smell  and  hear  and 

224 


THE  SPECIAL  SENSES  225 

see,  we  shall  have  to  base  all  our  study  on  our  own  experi- 
ence. We  know  of  hearing  and  seeing  only  by  what  we 
know  of  our  own  hearing  and  seeing ;  but  by  examination 
of  the  structure  of  the  hearing  and  seeing  organs  of  cer- 
tain other  animals,  and  by  observation  and  experiments, 
zoologists  are  convinced  that  some  animals  hear  sounds 
that  we  can  not  hear,  and  some  see  colors  that  we  can 
not  see. 

While  that  phase  .of  the  study  of  the  special  senses 
which  concerns  their  structure  may  be  quite  successfully 
undertaken,  the  physiological  phase  of  the  study  of  the 
actual  tasting  and  seeing  and  hearing  of  the  lower  animals 
is  a  matter  of  much  difficulty.  The  condition  and  char- 
acter of  the  special  senses  vary  notably  among  different 
animals.  There  may  even  exist  other  special  senses  than 
the  ones  we  possess.  Some  zoologists  believe  that  certain 
marine  animals  possess  a  "  density  or  pressure  sense  " — 
that  is,  a  sense  which  enables  them  to  tell  approximately 
how  deep  in  the  water  they  may  be  at  any  time.  To 
certain  animals  is  ascribed  a  "  temperature  sense,"  and 
some  zoologists  believe  that  what  we  call  the  homing  in- 
stinct of  animals  as  shown  by  the  homing  pigeons  and 
honey-bees  and  other  animals,  depends  on  their  possession 
of  a  special  sense  which  man  does  not  possess.  Eecent 
experiments,  however,  seem  to  show  that  the  homing  of 
pigeons  depends  on  their  keen  sight.  In  numerous  animals 
there  exist,  besides  the  organs  of  the  five  special  senses 
which  we  possess,  organs  whose  structure  compels  us  to  be- 
lieve them  to  be  organs  of  special  sense,  but  whose  func- 
tion is  wholly  unknown  to  us.  Thus  in  the  study  of  the 
special  senses  we  are  made  to  see  plainly  that  we  can  not 
rely  simply  on  our  knowledge  of  our  own  body  structure 
for  an  understanding  of  the  structure  and  functions  of 
other  animals. 

118.  Special  senses  of  the  simplest  animals. — In  the  Amcela 
(see  Chapter  I),  that  type  of  the  simplest  animals,  with 
16 


226  ANIMAL  LIFE 

one-celled  body,  without  organs,  and  yet  with  its  capacity 
for  performing  the  necessary  life  processes,  there  are  no 
special  senses  except  one  (perhaps  two).  The  Amoeba  can 
feel.  It  possesses  the  tactile  sense.  And  there  are  no 
special  sense  organs  except  one,  which  is  the  whole  of  the 
outer  surface  of  the  body.  If  the  Amceba  be  touched  with 
a  fine  point  it  feels  the  touch,  for  the  soft  viscous  proto- 
plasm of  its  body  flows  slowly  away  from  the  foreign  ob- 
ject. The  sense  of  feeling  or  touch,  the  tactile  sense,  is 
the  simplest  or  most  primitive  of  the  special  senses,  and 
the  simplest,  most  primitive  organ  of  special  sense  is  the 
outer  surface  or  skin  of  the  body.  Among  those  simple 
animals  that  possess  the  simplest  organs  of  hearing  and 
perceiving  light,  we  shall  find  these  organs  to  be  simply 
specialized  parts  of  the  skin  or  outer  cell  layer  of  the 
body,  and  it  is  a  fact  that  all  the  special  sense  organs  of 
all  animals  are  derived  or  developed  from  the  outer  cell 
layer,  ectoblast,  of  the  embryo.  This  is  true  also  of  the 
whole  nervous  system,  the  brain  and  spinal  cord  of  the 
vertebrates,  and  the  ganglia  and  nerve  commissures  of 
the  invertebrates.  And  while  in  the  higher  animals  the 
nervous  system  lies  underneath  the  surface  of  the  body, 
in  many  of  the  lower,  many-celled  animals  all  the  ganglia 
and  nerves,  all  of  the  nervous  system,  lie  on  the  outer 
surface  of  the  body,  being  simply  a  specialized  part  of 
the  skin. 

119.  The  sense  of  touch. — In  some  of  the  lower,  many- 
celled  animals,  as  among  the  polyps,  there  are  on  the  skin 
certain  sense  cells,  either  isolated  or  in  small  groups,  which 
seem  to  be  stimulated  not  alone  by  the  touching  of  foreign 
substances,  but  also  by  warmth  and  light.  They  are  not 
limited  to  a  single  special  sense.  They  are  the  primitive 
or  generalized  organs  of  special  sense,  and  can  develop  into 
specialized  organs  for  any  one  of  the  special  senses. 

The  simplest  and  most  widespread  of  these  special 
senses  with,  as  a  whole,  the  simplest  organs,  is  the  tactile 


THE  SPECIAL  SENSES 


227 


sense,  or  the  sense  of  touch.  The  special  organs  of  this 
sense  are  usually  simple  hairs  or  papillae  connecting  with  a 
nerve.  These  tactile  hairs  or  papillae  may  be  distributed 
pretty  evenly  over  most  of  the  body,  or  may  be  mainly  con- 
centrated upon  certain  parts  in  crowded  groups.  Many  of 
the  lower  animals  have  projecting  parts,  like  the  feeling 
tentacles  of  many  marine  invertebrates,  or  the  antennae 
(feelers)  of  crabs  and  insects,  which  are  the  special  seat 
of  the  tactile  organs.  Among  the  vertebrates  the  tactile 
organs  are  either  like  those  of  the  invertebrates,  or  are 
little  sac-like  bodies  of  connective  tissue  in  which  the 
end  of  a  nerve  is  curiously  folded  and  convoluted  (Fig. 
141).  These  little  touch  corpuscles  simply  lie  in  the  cell 
layer  of  the  skin,  covered  over  thinly  by  the  cuticle.  Some- 
times they  are  simply  free,  branched 
nerve-endings  in  the  skin.  These 
tactile  corpuscles  or  free  nerve-end- 
ings are  especially  abundant  in  those 
parts  of  the  body  which  can  be  best 
used  for  feeling.  In  man  the  fin- 
ger-tips are  thus  especially  supplied ; 
in  certain  tailed  monkeys  the  tip  of 
the  tail,  and  in  hogs  the  end  of  the 
snout.  The  difference  in  abundance 
of  these  tactile  corpuscles  of  the  skin 
can  be  readily  shown  by  experiment. 
With  a  pair  of  compasses,  whose  FlG;  .i«-Tactiie  papilla  of 

•f;  '  skin  of  man.     n,  nerve.— 

points  have  been  slightly  blunted,       After  KOELLIKEB. 
touch  the  skin  of  the  forearm  of  a 

person  who  has  his  eyes  shut,  with  the  points  about  three 
inches  apart  and  in  the  direction  of  the  length  of  the  arm. 
The  person  touched  will  feel  the  points  as  two.  Eepeat 
the  touching  several  times,  gradually  lessening  the  dis- 
tance between  the  points.  When  the  points  are  not  more 
than  an  inch  to  an  inch  and  a  half  apart,  the  person 
touched  will  feel  but  a  single  touch — that  is,  the  touching 


228  ANIMAL  LIFE 

of  both  points  will  give  the  sensation  of  but  a  single  con- 
tact. Eepeat  the  experiment  on  the  tip  of  the  forefinger, 
and  both  points  will  be  felt  until  the  points  are  only  about 
one  tenth  of  an  inch  apart. 

120.  The  sense  of  taste. — The  sense  of  taste  enables  us  to 
test  in  some  degree  the  chemical  constitution  of  substances 
which  are  taken  into  the  mouth  as  food.  We  discriminate 
by  the  taste  organs  between  good  food  and  bad,  well-tasting 
and  ill-tasting.  These  organs  are,  with  us  and  the  other  air- 
breathing  animals,  located  in  the  mouth  or  on  the  mouth 
parts.  They  must  be  located  so  as  to  come  into  contact 
with  the  food,  and  it  is  also  necessary  that  the  food  sub- 
stance to  be  tasted  be  made  liquid.  This  is  accomplished 
by  the  fluids  poured  into  the  mouth  from  the  salivary 
glands.  With  the  lower  aquatic  animals  it  is  not  improb- 
able that  taste  organs  are  situated  on  other  parts  of  the 
body  besides  the  mouth,  and  that  taste  is  used  not  only  to 
test  food  substances,  but  also  to  test  the  chemical  char- 
acter of  the  fluid  medium  in  which  they  live. 

The  taste  organs  are  much  like  the  tactile  organs,  ex- 
cept that  the  special  taste  cell  is  exposed,  so  that  small  par- 
ticles of  the  substance  to  be  tasted  can  come  into  actual 
contact  with  it.  The  nerve-ending  is  usually  in  a  small 
raised  papilla  or  depressed  pit.  In  the  simplest  animals 
there  is  no  special  organ  of  taste,  and  yet  Amoeba  and 
other  Protozoa  show  that  they  appreciate  the  chemical  con- 
stitution of  the  liquid  in  which  they  lie.  They  taste — that 
is,  test  the  chemical  constitution  of  the  substances — by 
means  of  their  undiiferentiated  body  surface.  The  taste 
organs  are  not  always  to  be  told  from  the  organs  of  smell. 
Where  an  animal  has  a  certain  special  seat  of  smell,  like 
the  nose  of  the  higher  animals,  then  the  special  sense 
organs  of  the  mouth  can  be  fairly  assumed  to  be  taste 
organs ;  but  where  the  seat  of  both  smell  and  taste  is  in 
the  mouth  or  mouth  parts,  it  is  often  impossible  to  distin- 
guish between  the  two  kinds  of  organs. 


THE  SPECIAL  SENSES 


In  mammals  taste  organs  are  situated  on  certain  parts  of 
the  tongue,  and  have  the  form  of  rather  large,  low,  broad 
papillae,  each  bearing  many  small  taste-buds  (Fig.  142). 
In  fishes  similar  papillae  and  buds  have  been  found  in  vari- 
ous places  on  the  sur- 
face of  the  body,  from 
which  it  is  believed  that 
the  sense  of  taste  in 
fishes  is  not  limited  to 
the  mouth.  In  insects 
the  taste  -  papillae  and 
taste -pits  are  grouped 

in  Certain  places  On  the  FIG.  14S.— Vertical  section  of  large  papilla  on 
mouth  parts,  being  es-  £^£of  a  Calf;  '•*••  taste-buds. -After 

pecially    abundant    on 

the  tips  of  small,  segmented,  feeler-like  processes  called 
palpi,  which  project  from  the  under  lip  and  from  the  so- 
called  maxillae. 

121.  The  sense  of  smell. — Smelling  and  tasting  are  closely 
allied,  the  one  testing  substances  dissolved,  the  other  test- 
ing substances  vaporized.  The  organs  of  the  sense  of 
smell  are,  like  those  of  taste,  simple  nerve-endings  in  papil- 
lae or  pits.  The  substance  to  be  smelled  must,  however, 
be  in  a  very  finely  divided  form ;  it  must  come  to  the  or- 
gans of  smell  as  a  gas  or  vapor,  and  not,  as  to  the  organs  of 
taste,  in  liquid  condition.  The  organs  of  smell  are  situated 
usually  on  the  head,  but  as  the  sense  of  smell  is  used  not 
alone  for  the  testing  of  food,  but  for  many  other  purposes, 
the  organs  of  smell  are  not,  like  those  of  taste,  situated 
principally  in  or  near  the  mouth.  Smell  is  a  special  sense 
of  much  wider  range  of  use  than  taste.  By  smell  animals 
can  discover  food,  avoid  enemies,  and  find  their  mates. 
They  can  test  the  air  they  breathe  as  well  as  the  food  they 
eat.  In  the  matter  of  the  testing  of  food  the  senses  of 
both  taste  and  smell  are  constantly  used,  and  are  indeed 
intimately  associated. 


230 


ANIMAL  LIFE 


The  sense  of  smell  varies  a  great  deal  in  its  degree  of 
development  in  various  animals.  With  the  strictly  aquatic 
animals — and  these  include  most  of  the  lower  invertebrates, 
as  the  polyps,  the  star-fishes,  sea-urchins,  and  most  of  the 
worms  and  mollusks — the  sense  of  smell  is  probably  but 
little  developed.  There  is  little  opportunity  for  a  gas  or 
vapor  to  come  to  these  animals,  and  only  as  a  gas  or  vapor 
can  a  substance  be  smelled.  With  these  animals  the  sense 
of  taste  must  take  the  place  of  the  olfactory  sense.  But 
among  the  insects,  mostly  terrestrial  animals,  there  is  an 
extraordinary  development  of  the  sense  of  smell.  It  is  in- 
deed probably  their  principal  special  sense.  Insects  must 
depend  on  smell  far  more  than  on  sight  or  hearing  for 
the  discovery  of  food,  for  becoming 
aware  of  the  presence  of  their  enemies 
and  of  the  proximity  of  their  mates 
and  companions.  The  organs  of 
smell  of  insects  are  situated  princi- 
pally on  the  antennae  or  feelers,  a 
single  pair  of  which  is  borne  on  the 
head  of  every  insect  (Fig.  143).  That 
many  insects  have  an  extraordinarily 
keen  sense  of  smell  has  been  shown 
by  numerous  experiments,  and  is  con- 
stantly proved  by  well-known  habits. 
If  a  small  bit  of  decaying  flesh  be  in- 
closed in  a  box  so  that  it  is  wholly 
concealed,  it  will  nevertheless  soon 
eating  beetle,  showing  be  found  by  the  flies  and  carrion 
^etles  that  either  feed  on  carrion 
or  must  always  lay  their  eggs  in  de- 
caying matter  so  that  their  carrion-eating  larvae  may  be 
provided  with  food.  It  is  believed  that  ants  find  their 
way  back  to  their  nests  by  the  sense  of  smell,  and  that 
they  can  recognize  by  scent  among  hundreds  of  individ- 
uals taken  from  various  communities  the  members  of  their 


THE  SPECIAL  SENSES 


231 


own  community.  In  the  insectary  at  Cornell  University, 
a  few  years  ago,  a  few  females  of  the  beautiful  promethea 
moth  (Callosamia  promethea)  were  inclosed  in  a  box, 
which  was  kept  inside  the  insectary  building.  No  males 
had  been  seen  about  the  insectary  nor  in  its  immediate 
vicinity,  although  they  had  been  sought  for  by  collectors. 
A  few  hours  after  the  beginning  of  the  captivity  of  the 
female  moths  there  were  forty  male  prometheas  fluttering 
about  over  the  glass  roof  of  the  insectary.  They  could  not 


FIG.  144.— Promethea  moth,  male,  showing  specialized  antennae. 

see  the  females,  and  yet  had  discovered  their  presence  in 
the  building.  The  discovery  was  undoubtedly  made  by  the 
sense  of  smell.  These  moths  have  very  elaborately  devel- 
oped antennae  (Fig.  144),  finely  branched  or  feathered, 
affording  opportunity  for  the  existence  of  very  many  smell- 
ing-pits. 

The  keenness  of  scent  of  hounds  and  bird  dogs  is  famil- 
iar to  all,  although  ever  a  fresh  source  of  astonishment  as 
we  watch  these  animals  when  hunting.  We  recently 
watched  a  retriever  dog  select  unerringly,  by  the  sense  of 
smell,  any  particular  duck  out  of  a  pile  of  a  hundred.  In 


232  ANIMAL  LIFE 

the  case  of  man  the  sense  of  smell  is  not  nearly  so  well 
developed  as  among  many  of  the  other  vertebrates.  This 
inferiority  is  largely  due  to  degeneration  through  lessened 
need;  for  in  Indians  and  primitive  races  the  sense  of 
smell  is  keener  and  better  developed  than  in  civilized 
races.  Where  man  has  to  make  his  living  by  hunting,  and 
has  to  avoid  his  enemies  of  jungle  and  plain,  his  special 
senses  are  better  developed  than  where  the  necessity  of 
protection  and  advantage  by  means  of  such  keenness  of 
scent  and  hearing  is  done  away  with  by  the  arts  of  civi- 
lization. 

122.  The  sense  of  hearing. — Hearing  is  the  perception 
of  certain  vibrations  of  bodies.  These  vibrations  give  rise 
to  waves — sound  waves  as  they  are  called — which  proceed 
from  the  vibrating  body  in  all  directions,  and  which,  com- 
ing to  an  animal,  stimulate  the  special  auditory  or  hearing 
organs,  that  transmit  this  stimulation  along  the  auditory 
nerve  to  the  brain,  where  it  is  translated  as  sound.  These 
sound  waves  come  to  animals  usually  through  the  air,  or, 
in  the  case  of  aquatic  animals,  through  water,  or  through 
both  air  and  water. 

The  organs  of  hearing  are  of  very  complex  structure 
in  the  case  of  man  and  the  higher  vertebrates.  Our  ears, 
which  are  adapted  for  perceiving  or  being  stimulated  by 
vibrations  ranging  from  16  to  40,000  a  second — that  is,  for 
hearing  all  those  sounds  produced  by  vibrations  of  a  rapid- 
ity not  less  than  16  to  a  second  nor  greater  than  40,000  to 
a  second — are  of  such  complexity  of  structure  that  many 
pages  would  be  required  for  their  description.  But  among 
the  lower  or  less  highly  organized  animals  the  ears,  or  au- 
ditory organs,  are  much  simpler. 

In  most  animals  the  auditory  organs  show  the  common 
characteristic  of  being  wholly  composed  of,  or  having  as 
an  essential  part,  a  small  sac  filled  with  liquid  in  which 
one  or  more  tiny  spherical  hard  bodies  called  otoliths  are 
held.  This  auditory  sac  is  formed  of  or  lined  internally  by 


THE  SPECIAL  SENSES 


233 


auditory  cells,  specialized  nerve  cells,  which  often  bear 
delicate  vibratile  hairs  (Fig.  145).  Auditory  organs  of  this 
general  character  are  known  among  the  polyps,  the  worms, 
the  crustaceans,  and  the  mollusks.  In  the  common  cray- 
fish the  "  ears "  are  situated  in  the  basal  segment  of  the 
inner  antennae  or  feelers  (Fig.  14G).  They  consist  each  of 
a  small  sac  filled  with  liquid  in  which 
are  suspended  several  grains  of  sand 
or  other  hard  bodies.  The  inner 


FIG.  145. — Auditory  organ  of  a  mollusk.  a,  audi- 
tory nerve ;  b,  outer  wall  of  connective  tissue ; 
c,  cells  with  auditory  hairs ;  d,  otolith.— After 
LEYDIG. 


PIG.  146.  —  Antenna  of 
cray  -  fish,  with  audi- 
tory sac  at  base. — 
After  HUXLEY. 


surface  of  the  sac  is  lined  with  fine  auditory  hairs.  The 
sound  waves  coming  through  the  air  or  water  outside  strike 
against  this  sac,  which  lies  in  a  hollow  on  the  upper  or 
outer  side  of  the  antennae.  The  sound  waves  are  taken  up 
by  the  contents  of  the  sac  and  stimulate  the  fine  hairs, 
which  in  turn  give  this  stimulus  to  the  nerves  which  run 
from  them  to  the  principal  auditory  nerve  and  thus  to  the 
brain  of  the  cray-fish.  Among  the  insects  'other  kinds  of 
auditory  organs  exist.  The  common  locust  or  grasshopper 


234 


ANIMAL  LIFE 


has  on  the  upper  surface  of  the  first  abdominal  segment 
a  pair  of  tympana  or  ear-drums  (Fig.  147),  composed  sim- 
ply of  the  thinned,  tightly  stretched   chitinous 
cuticle  of  the  body.     On  the  inner  surface  of  this 


FIG.  147.— Grasshopper,  showing  auditory  organ  (a.  0.)  in  first  segment  of  abdomen. 
(Wings  of  one  side  removed.) 

ear-drum  there  are  a  tiny  auditory  sac,  a  fine  nerve  lead- 
ing from  it  to  a  small  auditory  ganglion  lying  near  the 
tympanum,  and  a  large  nerve  leading  from  this  ganglion 
to  one  of  the  larger  ganglia  situated  on  the  floor  of  the 


a.o 

FIG.  148.— A  cricket,  showing  auditory  organ  (a.  o.)  in  fore-leg. 

thorax.  In  the  crickets  and  katydids,  insects  related  to 
the  locusts,  the  auditory  organs  or  ears  are  situated  in  the 
fore-legs  (Fig.  148). 

Certain  other  insects,  as  the  mosquitoes  and  other  midges 


OF  THE  ^ 

UNIVERSITY  ) 

OF 


THE  SPECIAL  SENSES 


235 


or  gnats,  undoubtedly  hear  by  means  of  numerous  delicate 

hairs  borne  on  the  antennae.     The  male  mosquitoes  (Fig. 

149)  have  many  hundreds  of  these  long,  fine  antennal  hairs, 

and  on  the  sounding  of  a  tuning-fork  these  hairs  have  been 

observed  to  vibrate  strongly.     In  the  base  of  each  antenna 

there  is  a  most  elaborate  organ, 

composed    of    fine     chitinous 

rods,  and  accompanying  nerves 

and  nerve  cells  whose  function 

it  is  to  take  up  and  transmit 

through  the  auditory  nerve  to 

the  brain  the  stimuli  received 

from  the  external  auditory 

hairs. 

123.  Sound  -making.  —  The 
sense  of  hearing  enables  ani- 
mals not  only  to  hear  the 
warning  natural  sounds  of 
storms  and  falling  trees  and 
plunging  avalanches,  but  the 
sounds  made  by  each  other. 
Sound-making  among  animals 
serves  to  aid  in  frightening 
away  enemies  or  in  warning 
companions  of  their  approach, 
for  recognition  among  mates 

and  members  of  a  band  or  species,  for  the  attracting  and 
wooing  of  mates,  and  for  the  interchange  of  information. 
With  the  cries  and  roars  of  mammals,  the  songs  of  birds, 
and  the  shrilling  and  calling  of  insects  all  of  us  are  familiar. 
These  are  all  sounds  that  can  be  heard  by  the  human  ear. 
But  that  there  are  many  sounds  made  by  animals  that 
we  can  not  hear — that  is,  that  are  of  too  high  a  pitch  for 
our  hearing  organs  to  be  stimulated  by — is  believed  by  nat- 
uralists. Especially  is  this  almost  certainly  true  in  the  case 
of  the  insects.  The  peculiar  sound-producing  organs  of 


FIG.  149. — A  male  mosquito,  showing 
auditory  hairs  (a.  h.)  on  the  an- 
tennae. 


236  ANIMAL  LIFE 

many  sound-making  insects  are  known ;  but  certain  other 
insects,  which  make  no  sound  that  we  can  hear,  neverthe- 
less possess  similar  sound-making  organs. 

Sound  is  produced  by  mammals  and  birds  by  the  strik- 
ing of  the  air  which  goes  to  and  comes  from  the  lungs 
against  certain  vibratory  cords  or  flaps  in  the  air-tubes. 
Sounds  made  by  this  vibration  are  re-enforced  and  made 
louder  by  arrangements  of  the  air-tubes  and  mouth  for 
resonance,  and  the  character  or  quality  of  the  sound  is 
modified  at  will  to  a  greater  or  less  degree  by  the  lips  and 
teeth  and  other  mouth  structures.  Sounds  so  made  are 
said  to  be  produced  by  a  voice,  or  animals  making  sounds 
in  this  way  are  said  to  possess  a  voice.  Animals  possessing 
a  voice  have  far  more  range  and  variety  in  their  sound- 
making  than  most  of  the  animals  which  produce  sounds  in 
other  ways.  The  marvelous  variety  and  the  great  strength 
of  the  singing  of  birds  and  of  the  cries  and  roars  of  mam- 
mals are  unequaled  by  the  sounds  of  any  other  animals. 

But  many  animals  without  a  voice — that  is,  which  do  not 
make  sounds  from  the  air-tubes — make  sounds,  and  some 
of  them,  as  certain  insects,  show  much  variety  and  range 
in  their  singing.  The  sounds  of  insects  are  made  by  the 
rapid  vibrations  of  the  wings,  as  the  humming  or  buzzing 
of  bees  and  flies,  by  the  passage  of  air  out  or  into  the  body 
through  the  many  breathing  pores  or  spiracles  (a  kind 
of  voice),  by  the  vibration  of  a  stretched  membrane  or 
tympanum,  as  the  loud  shrilling  of  the  cicada,  and  most 
commonly  by  stridulation — that  is,  by  rubbing  together 
two  roughened  parts  of  the  body.  The  male  crickets  and 
the  male  katydids  rub  together  the  bases  of  their  wing 
covers  to  produce  their  shrill  singing.  The  locusts  or 
grasshoppers  make  sounds  when  at  rest  by  rubbing  the 
roughened  inside  of  their  great  leaping  legs  against  the 
upper  surface  of  their  wing  covers,  and  when  in  flight  by 
striking  the  two  wings  of  each  side  together.  Numerous 
other  insects  make  sounds  by  stridulation,  but  many  of 


THE  SPECIAL  SENSES  237 

these  sounds  are  so  feeble  or  so  high  in  pitch  that  they  are 
rarely  heard  by  us.  Certain  butterflies  make  an  odd  click- 
ing sound,  as  do  some  of  the  water-beetles.  In  Japan, 
where  small  things  which  are  beautiful  are  prized  not  less 
than  large  ones,  singing  insects  are  kept  in  cages  and 
highly  valued,  so  that  their  capture  becomes  a  lucrative 
industry,  just  as  it  is  with  song  birds  in  Europe  and  Amer- 
ica. Among  the  many  species  of  Japanese  singing  insects 
is  a  night  cricket,  known  as  the  bridle-bit  insect,  because 
its  note  resembles  the  jingling  of  a  bridle-bit. 

124.  The  sense  of  sight. — Rot  all  animals  have  eyes. 
The  moles  which  live  underground,  insects,  and  other  ani- 
mals that  live  in  caves,  and  the  deep-sea  fishes  which  live 
in  waters  so  deep  that  the  light  of  the  sun  never  comes 
to  them,  have  no  eyes  at  all,  or  have  eyes  of  so  rudimentary 
a  character  that  they  can  no  longer  be  used  for  seeing. 
But  all  these  eyeless  animals  have  no  eyes  because  they 
live  under  conditions  where  eyes  are  useless.  They  have 
lost  their  eyes  by  degeneration.  There  are,  however,  many 
animals  that  have  no  eyes,  nor  have  they  or  their  ancestors 
ever  had  eyes.  These  are  the  simplest,  most  lowly  organ- 
ized animals.  Many,  perhaps  all  eyeless  animals  are,  how- 
ever, capable  of  distinguishing  light  from  darkness.  They 
are  sensitive  to  light.  An  investigator  placed  several  indi- 
viduals of  the  common,  tiny  fresh- water  polyp  (Hydra)  in  a 
glass  cylinder  the  walls  of  which  were  painted  black.  He 
left  a  small  part  of  the  cylinder  unpainted,  and  in  this  part 
of  the  cylinder  where  the  light  penetrated  the  Hydras  all 
gathered.  The  eyeless  maggots  or  larvae  of  flies,  when 
placed  in  the  light  will  wriggle  and  squirm  away  into  dark 
crevices.  They  are  conscious  of  light  when  exposed  to  it, 
and  endeavor  to  shun  it.  Most  plants  turn  their  leaves 
toward  the  light ;  the  sunflowers  turn  on  their  stems  to 
face  the  sun.  Light  seems  to  stimulate  organisms  whether 
they  have  eyes  or  not,  and  the  organisms  either  try  to  get 
into  the  light  or  to  avoid  it.  But  this  is  not  seeing. 


238 


ANIMAL  LIFE 


The  simplest  eyes,  if  we  may  call  them  eyes,  are  not 
capable  of  forming  an  image  or  picture  of  external  objects. 
They  only  make  the  animal  better  capable  of  distinguish- 
ing between  light  and  darkness  or  shadow.  Many  lowly 
organized  animals,  as  some  polyps,  and  worms,  have  certain 
cells  of  the  skin  specially  provided  with  pigment.  These 
cells  grouped  together  form  what  is  called  a  pigment  fleck, 
which  can,  because  of  the  presence  of  the  pigment,  absorb 
more  light  than  the  skin  cells,  and  are  more  sensitive  to 
the  light.  By  such  pigment-flecks,  or  eye-spots,  the  animal 
can  detect,  by  their  shadows,  the  passing  near  them  of  mov- 
ing bodies,  and  thus  be  in  some  measure  informed  of  the 
approach  of  enemies  or  of  prey.  Some  of  these  eye-flecks 
are  provided,  not  simply  with  pigment,  but  with  a  simple 
sort  of  lens  that  serves  to  concentrate  rays  of  light  and 

make  this  simplest 
sort  of  eye  even 
more  sensitive  to 
changes  in  the  in- 
tensity of  light 
(Fig.  150). 

Most  of  the 
many  -  celled  ani- 
mals possess  eyes 
by  means  of  which 
a  picture  of  exter- 
nal objects  more  or  less  nearly  complete  and  perfect  can 
be  formed.  There  is  great  variety  in  the  finer  structure 
of  these  picture-forming  eyes,  but  each  consists  essentially 
of  an  inner  delicate  or  sensitive  nervous  surface  called  the 
retina,  which  is  stimulated  by  light,  and  is  connected  with 
the  brain  by  a  large  optic  nerve,  and  of  a  transparent  light- 
refracting  lens  lying  outside  of  the  retina  and  exposed  to 
the  light.  These  are  the  constant  essential  parts  of  an 
image  -  forming  and  image -perceiving  eye.  In  most  eyes 
there  are  other  accessory  parts  which  may  make  the  whole 


FIG.  150.— The  simple  eye  of  a  jelly-fish  (Lizzia 
koettikeri)—MteT  O.  and  R.  HBRTWIO. 


THE  SPECIAL  SENSES 


239 


eye  an  organ  of  excessively  complicated  structure  and  of 
remarkably  perfect  seeing  capacity.  Our  own  eyes  are 
organs  of  extreme  structural  complexity  and  of  high  de- 
velopment, although  some  of  the  other  vertebrates  have 
undoubtedly  a  keener  and  more  nearly  perfected  sight. 

The  crustaceans  and  insects  have  eyes  of  a  peculiar 
character  called  compound  eyes.  In  addition  most  insects 
have  smaller  simple  eyes.  Each  of  the  compound  eyes  is 
composed  of  many  (from  a  few,  as  in  certain  ants,  to  as 
many  as  twenty-five  thousand,  as  in  certain  beetles)  eye  ele- 
ments, each  eye  element  seeing  independently  of  the  other 
eye  elements  and  seeing  only  a  very  small  part  of  any  ob- 
ject in  front  of  the  whole  eye.  All  these  small  parts  of 
the  external  object  seen  by  the  many  distinct  eye  elements 
are  combined  so  as  to  form  an  image  in  mosaic — that  is, 
made  up  of  separate  small  parts — of  the  external  object. 
^^  If  the  head  of  a  dragon-fly  be  exam- 

Ov  ined,  it  will  be  seen  that 

x  two  thirds  or  more  of  the 


FIG.  151. — A  dragon-fly,  showing  the  large  com- 
pound eyes  on  the  head. 


PIG.  152.— Some  of  the  facets 
of  the  compound  eye  of  a 
dragon-fly. 


whole  head  is  made  up  of  the  two  large  compound  eyes 
(Fig.  151),  and  with  a  lens  it  may  be  seen  that  the  outer 
surface  of  each  of  these  eyes  is  composed  of  many  small 
spaces  or  facets  (Fig.  152)  which  are  the  outer  lenses  of 
the  many  eye  elements  composing  the  whole  eye. 


CHAPTEE  XIV 

INSTINCT   AND   REASON 

125.  Irritability. — All  animals   of  whatever  degree    of 
organization  show  in  life  the  quality  of  irritability  or  re- 
sponse to  external  stimulus.     Contact  with  external  things 
produces  some  effect  on  each  of  them,  and  this  effect  is 
something  more  than  the  mere  mechanical  effect  on  the 
matter  of  which  the  animal   is   composed.      In   the   one- 
celled  animals  the  functions  of  response  to  external  stimu- 
lus are  not  localized.     They  are  the  property  of  any  part  of 
the  protoplasm  of  the  body.     Just  as  breathing  or  digestion 
is  a  function  of  the  whole  cell,  so  are  sensation  and  response 
in  action.     In  the  higher  or  many-celled  animals  each  of 
these  functions  is  specialized  and  localized.     A  certain  set 
of  cells  is  set  apart  for  each  function,  and  each  organ  or 
series  of  cells  is  released  from  all  functions  save  its  own. 

126.  Nerve  cells  and  fibers. — In  the  development  of  the 
individual  animal  certain  cells  from  the  primitive  external 
layer  or  ectoblast  of  the  embryo  are  set  apart  to  preside 
over  the  relations   of  the   creature    to    its   environment. 
These  cells  are  highly  specialized,  and  while  some  of  them 
are  highly  sensitive,  others  are   adapted  for  carrying  or 
transmitting  the  stimuli  received  by  the  sensitive  cells,  and 
still  others  have  the  function  of  receiving  sense-impressions 
and  of  translating  them  into  impulses  of  motion.      The 
nerve  cells  are  receivers  of  impressions.     These  are  gathered 
together  in  nerve  masses  or  ganglia,  the  largest  of  these 
being  known  as  the  brain,  the  ganglia  in  general  being 
known  as  nerve  centers.     The  nerves  are  of  two  classes. 

240 


INSTINCT  AND  REASON  241 

The  one  class,  called  sensory  nerves,  extends  from  the  skin 
or  other  organ  of  sensation  to  the  nerve  center.  The  nerves 
of  the  other  class,  motor  nerves,  carry  impulses  to  motion. 

127.  The  brain  or  sensorium.— The  brain  or  other  nerve 
center  sits  in  darkness  surrounded  by  a  bony  protecting 
box.     To  this  main  nerve  center,  or  sensorium,  come  the 
nerves  from   all  parts   of  the  body  that   have   sensation, 
the  external  skin  as  well  as  the  special  organs  of  sight, 
hearing,  taste,  smell.     With  these  come  nerves  bearing  sen- 
sations of  pain,  temperature,  muscular  effort — all  kinds  of 
sensation  which  the  brain  can  receive.     These  nerves  are 
the  sole  sources  of  knowledge   to   any  animal   organism. 
Whatever  idea   its  brain   may  contain  must  be  built  up 
through  these  nerve  impressions.     The  aggregate  of  these 
impressions  constitute  the  world  as  the  organism  knows  it. 
All  sensation  is  related  to  action.     If  an  organism  is  not 
to  act,  it  can  not  feel,  and  the  intensity  of  its  feeling  is 
related  to  its  power  to  act. 

128.  Reflex  action. — These  impressions  brought  to  the 
brain  by  the  sensory  nerves  represent  in  some  degree  the 
facts  in  the  animal's  environment.     They  teach  something 
as  to  its  food  or  its  safety.     The  power  of  locomotion  is 
characteristic  of  animals.    If  they  move,  their  actions  must 
depend  on  the  indications  carried  to  the  nerve  center  from 
the  outside;  if  they  feed  on  living  organisms,  they  must 
seek  their  food ;  if,  as  in  many  cases,  other  living  organ- 
isms prey  on  them,  they  must  bestir  themselves  to  escape. 
The  impulse  of  hunger  on  the  one  hand  and  of  fear  on  the 
other  are  elemental.     The  sensorium  receives  an  impression 
that  food  exists  in  a  certain  direction.     At  once  an  impulse 
to  motion  is  sent  out  from  it  to  the  muscles  necessary  to 
move  the  body  in  that  direction.     In  the  higher  animals 
these  movements  are  more  rapid  and  more  exact.     This  is 
because  organs  of  sense,  muscles,  nerve  fibers,  and  nerve 
cells  are  all  alike  highly  specialized.     In  the  star-fish  the 
sensation  is  slow,  the  muscular  response  sluggish,  but  the 

17 


242  ANIMAL  LIFE 

method  remains  the  same.  This  is  simple  reflex  action,  an 
impulse  from  the  environment  carried  to  the  brain  and 
then  unconsciously  reflected  back  as  motion.  The  impulse 
of  fear  is  of  the  same  nature.  Strike  at  a  dog  with  a  whip, 
and  he  will  instinctively  shrink  away,  perhaps  with  a  cry. 
Perhaps  he  will  leap  at  you,  and  you  unconsciously  will  try 
to  escape  from  him.  Reflex  action  is  in  general  uncon- 
scious, but  with  animals  as  with  man  it  shades  by  degrees 
into  conscious  action,  and  into  volition  or  action  "  done  on 
purpose." 

129.  Instinct. — Different  one-celled  animals  show  differ- 
ences in  method  or  degree  of  response  to  external  influences. 
The  feelers  of  the  Amceba  will  avoid  contact  with  the  feel- 
ers or  pseudopodia  of  another  Amoeba,  while  it  does  not 
shrink  from  contact  with  itself  or  with  an  organism  of  un- 
like kind  on  which  it  may  feed.  Most  Protozoa  will  discard 
grains  of  sand,  crystals  of  acid,  or  other  indigestible  object. 
Such  peculiarities  of  different  forms  of  life  constitute  the 
basis  of  instinct. 

Instinct  is  automatic  obedience  to  the  demands  of  ex- 
ternal conditions.  As  these  conditions  vary  with  each  kind 
of  animal,  so  must  the  demands  vary,  and  from  this  arises 
the  great  variety  actually  seen  in  the  instincts  of  different 
animals.  As  the  demands  of  life  become  complex,  so  may 
the  instincts  become  so.  The  greater  the  stress  of  envi- 
ronment, the  more  perfect  the  automatism,  for  impulses  to 
safe  action  are  necessarily  adequate  to  the  duty  they  have 
to  perform.  If  the  instinct  were  inadequate,  the  species 
would  have  become  extinct.  The  fact  that  its  individuals 
persist  shows  that  they  are  provided  with  the  instincts 
necessary  to  that  end.  Instinct  differs  from  other  allied 
forms  of  response  to  external  condition  in  being  hereditary, 
continuous  from  generation  to  generation.  This  suffi- 
ciently distinguishes  it  from  reason,  but  the  line  between 
instinct  and  reason  and  other  forms  of  reflex  action  can 
not  be  sharply  drawn. 


INSTINCT  AND  REASON  243 

It  is  not  necessary  to  consider  here  the  question  of  the 
origin  of  instincts.  Some  writers  regard  them  as  "  inherited 
habits,"  while  others,  with  apparent  justice,  doubt  if  mere 
habits  or  voluntary  actions  repeated  till  they  become  a 
"  second  nature  "  ever  leave  a  trace  upon  heredity.  Such 
investigators  regard  instinct  as  the  natural  survival  of  those 
methods  of  automatic  response  which  were  most  useful  to 
the  life  of  the  animal,  the  individuals  having  less  effective 
methods  of  reflex  action  having  perished,  leaving  no  pos- 
terity. 

An  example  in  point  would  be  the  homing  instinct  of 
the  fur-seal.  When  the  arctic  winter  descends  on  its  home 
in  the  Pribilof  Islands  in  Bering  Sea,  these  animals  take 
to  the  open  ocean,  many  of  them  swimming  southward  as 
far  as  the  Santa  Barbara  Islands  in  California,  more  than 
three  thousand  miles  from  home.  While  on  the  long  swim 
they  never  go  on  shore,  but  in  the  spring  they  return  to 
the  northward,  finding  the  little  islands  hidden  in  the  arc- 
tic fogs,  often  landing  on  the  very  spot  from  which  they 
were  driven  by  the  ice  six  months  before,  and  their  arrival 
timed  from  year  to  year  almost  to  the  same  day.  The  per- 
fection of  this  homing  instinct  is  vital  to  their  life.  If 
defective  in  any  individual,  he  would  be  lost  to  the  herd 
and  would  leave  no  descendants.  Those  who  return  be- 
come the  parents  of  the  herd.  As  to  the  others  the  rough 
sea  tells  no  tales.  We  know  that,  of  those  that  set  forth,  a 
large  percentage  never  comes  back.  To  those  that  return 
the  homing  instinct  has  proved  adequate.  This  must  be  so 
so  long  as  the  race  exists.  The  failure  of  instinct  would 
mean  the  extinction  of  the  species. 

130.  Classification  of  instincts. — The  instincts  of  animals 
may  be  roughly  classified  as  to  their  relation  to  the  indi- 
vidual into  egoistic  and  altruistic  instincts. 

Egoistic  instincts  are  those  which  concern  chiefly  the 
individual  animal  itself.  To  this  class  belong  the  instincts 
of  feeding,  those  of  self-defense  and  of  strife,  the  instincts 


244  ANIMAL  LIFE 

of  play,  the  climatic  instincts,  and  environmental  instincts, 
those  which  direct  the  animal's  mode  of  life. 

Altruistic  instincts  are  those  which  relate  to  parent- 
hood and  those  which  are  concerned  with  the  mass  of  indi- 
viduals of  the  same  species.  The  latter  may  be  called  the 
social  instincts.  In  the  former  class,  the  instincts  of  par- 
enthood, may  be  included  the  instincts  of  courtship,  re- 
production, home-making,  nest-building,  and  care  for  the 
young. 

131.  Feeding. — The  instincts  of  feeding  are  primitively 
simple,  growing  complex  through  complex  conditions. 
The  protozoan  absorbs  smaller  creatures  which  contain 
nutriment.  The  sea-anemone  closes  its  tentacles  over  its 
prey.  The  barnacle  waves  its  feelers  to  bring  edible  crea- 
tures within  its  mouth.  The  fish  seizes  its  prey  by  direct 
motion.  The  higher  vertebrates  in  general  do  the  same, 
but  the  conditions  of  life  modify  this  simple  action  to  a 
very  great  degree. 

In  general,  animals  decide  by  reflex  actions  what  is 
suitable  food,  and  by  the  same  processes  they  reject  poisons 
or  unsuitable  substances.  The  dog  rejects  an  apple,  while 
the  horse  rejects  a  piece  of  meat.  Either  will  turn  away 
from  an  offered  stone.  Almost  all  animals  reject  poisons 
instantly.  Those  who  fail  in  this  regard  in  a  state  of 
nature  die  and  leave  no  descendants.  The  wild  vetches  or 
"  loco-weeds  "  of  the  arid  regions  affect  the  nerve  centers  of 
animals  and  cause  dizziness  or  death.  The  native  ponies 
reject  these  instinctively.  This  may  be  because  all  ponies 
which  have  not  this  reflex  dislike  have  been  destroyed. 
The  imported  horse  has  no  such  instinct  and  is  poisoned. 
Very  few  animals  will  eat  any  poisonous  object  with  which 
their  instincts  are  familiar,  unless  it  be  concealed  from  smell 
and  taste. 

In  some  cases,  very  elaborate  instincts  arise  in  connec- 
tion with  feeding  habits.  With  the  California  woodpeckers 
(Melanerpes  formicivorus  lairdii)  a  large  number  of  them 


INSTINCT  AND  REASON  245 

together  select  a  live-oak  tree  for  their  operations.  They 
first  bore  its  bark  full  of  holes,  each  large  enough  to  hold 
an  acorn.  Then  into  each  hole  an  acorn  is  thrust  (Figs. 
61  and  62).  Only  one  tree  in  several  square  miles  may  be 
selected,  and  when  their  work  is  finished  all  those  inter- 
ested go  about  their  business  elsewhere.  At  irregular  in- 
tervals a  dozen  or  so  come  back  with  much  clamorous  dis- 
cussion to  look  at  the  tree.  When  the  right  time  comes, 
they  all  return,  open  the  acorns  one  by  one,  devouring 
apparently  the  substance  of  the  nut,  and  probably  also  the 
grubs  of  beetles  which  have  developed  within.  When  the 
nuts  are  ripe,  again  they  return  to  the  same  tree  and  the 
same  process  is  repeated.  In  the  tree  figured  this  has  been 
noticed  each  year  since  1891. 

132.  Self-defense. — The  instinct  of  self-defense  is  even 
more  varied  in  its  manifestations.  It  may  show  itself 
either  in  the  impulse  to  make  war  on  an  intruder  or  in  the 
desire  to  flee  from  its  enemies.  Among  the  flesh-eating 
mammals  and  birds  fierceness  of  demeanor  serves  both  for 
the  securing  of  food  and  for  protection  against  enemies. 
The  stealthy  movements  of  the  lion,  the  skulking  habits  of 
the  wolf,  the  sly  selfishness  of  the  fox,  the  blundering  good- 
natured  power  of  the  bear,  the  greediness  of  the  hyena,  are 
all  proverbial,  and  similar  traits  in  the  eagle,  owl,  hawk, 
and  vulture  are  scarcely  less  matters  of  common  observa- 
tion. 

Herbivorous  animals,  as  a  rule,  make  little  direct  resist- 
ance to  their  enemies,  depending  rather  on  swiftness  of 
foot,  or  in  some  cases  on  simple  insignificance.  To  the  lat- 
ter cause  the  abundance  of  mice  and  mouse-like  rodents 
may  be  attributed,  for  all  are  the  prey  of  carnivorous  beasts 
and  birds,  and  even  snakes. 

Even  young  animals  of  any  species  show  great  fear  of 
their  hereditary  enemies.  The  nestlings  in  a  nest  of  the 
American  bittern  when  one  week  old  showed  no  fear  of 
man,  but  when  two  weeks  old  this  fear  was  very  manifest 


246 


ANIMAL  LIFE 


(Figs.  153  and  154).  Young  mocking-birds  will  go  into 
spasms  at  the  sight  of  an  owl  or  a  cat,  while  they  pay  little 
attention  to  a  dog  or  a  hen.  Monkeys  that  have  never 
seen  a  snake  show  almost  hysterical  fear  at  first  sight  of 
one,  and  the  same  kind  of  feeling  is  common  to  most 
men.  A  monkey  was  allowed  to  open  a  paper  bag  which 


FlQ.  153.— Nestlings  of  the  American  bittern.  Two  of  a  brood  of  four  birds  one  week 
old,  at  which  age  they  showed  no  fear  of  man.  Photograph  by  E.  H.  TABOR, 
Meridian,  N.  Y.,  May  31,  1898.  (Permission  of  Macmillan  Company,  publishers  of 
Bird-Lore.) 

contained  a  live  snake.  He  was  staggered  by  the  sight, 
but  after  a  while  went  back  and  looked  in  again,  to  repeat 
the  experience.  Each  wild  animal  has  its  special  instinct 
of  resistance  or  method  of  keeping  off  its  enemies.  The 
stamping  of  a  sheep,  the  kicking  of  a  horse,  the  running 
in  a  circle  of  a  hare,  and  the  skulking  in  a  circle  of  some 
foxes,  are  examples  of  this  sort  of  instinct. 


INSTINCT  AND  REASON 


247 


133.  Play. — The  play  instinct  is  developed  in  numerous 
animals.  To  this  class  belong  the  wrestlings  and  mimic 
fights  of  young  dogs,  bear  cubs,  seal  pups,  and  young 
beasts  generally.  Cats  and  kittens  play  with  mice.  Squir- 


FIG.  154.— Nestlings  of  the  American  bittern.  The  four  members  of  the  brood  of 
which  two  are  shown  in  Fig.  153,  two  weeks  old,  when  they  showed  marked  fear 
of  man.  Photograph  by  F.  M.  CHAPMAN,  Meridian,  N.  Y.,  June  8,  1898.  (Per- 
mission of  Macmillan  Company,  publishers  of  Bird-Lore.) 

rels  play  in  the  trees.  Perhaps  it  is  the  play  impulse  which 
leads  the  shrike  or  butcher-bird  to  impale  small  birds  and 
beetles  on  the  thorns  about  its  nest,  a  ghastly  kind  of  orna- 
ment that  seems  to  confer  satisfaction  on  the  bird  itself. 
The  talking  of  parrots  and  their  imitations  of  the  sounds 
they  hear  seem  to  be  of  the  nature  of  play.  The  greater 


248  ANIMAL  LIFE 

their  superfluous  energy  the  more  they  will  talk.  Much  of 
the  singing  of  birds,  and  the  crying,  calling,  and  howling  of 
other  animals,  are  mere  play,  although  singing  primarily  be- 
longs to  the  period  of  reproduction,  and  other  calls  and 
cries  result  from  social  instincts  or  from  the  instinct  to 
care  for  the  young. 

134.  Climate. — Climatic  instincts  are  those  which  arise 
from  the  change  of  seasons.     When  the  winter  comes  the 
fur-seal  takes  its  long  swim  to  the  southward;  the  wild 
geese  range  themselves  in  wedge-shaped  nocks  and  fly  high 
and  far,  calling  loudly  as  they  go ;  the  bobolinks  straggle 
away  one  at  a  time,  flying  mostly  in  the  night,  and  most  of 
the  smaller  birds  in  cold  countries  move  away  toward  the 
tropics.     All  these  movements  spring  from  the  migratory 
instinct.     Another  climatic  instinct  leads  the  bear  to  hide 
in  a  cave  or  hollow  tree,  where  he  sleeps  or  hibernates  till 
spring.     In  some  cases  the  climatic  instinct  merges  in  the 
homing  instinct  and  the  instinct  of  reproduction.     When 
the  birds  move  north  in  the  spring  they  sing,  mate,  and 
build  their  nests.     The  fur-seal  goes  home  to  rear  its  young. 
The  bear  exchanges  its  bed  for  its  lair,  and  its  first  business 
after  waking  is  to  make  ready  to  rear  its  young. 

135.  Environment. — Environmental     instincts     concern 
the  creature's  mode  of  life.    Such  are  the  burrowing  instincts 
of  certain  rodents,  the  woodchucks,  gophers,  and  the  like. 
To  enumerate  the  chief  phases  of  such  instincts  would  be 
difficult,  for  as  all  animals  are   related  to  their  environ- 
ment, this  relation  must  show  itself  in  characteristic  in- 
stincts. 

136.  Courtship. — The  instincts  of  courtship  relate  chiefly 
to  the  male,  the  female  being  more  or  less  passive.     Among 
many  fishes  the  male  struts  before  the  female,  spreading 
his  fins,  intensifying  his  pigmented  colors  through  muscu- 
lar tension,  and  in  such  fashion  as  he  can  makes  himself  the 
preferred  of  the  female.     In  the  little  brooks  in   spring 
male  minnows  can  be  found  with  warts  on  the  nose  or  head, 


INSTINCT  AND  REASON  249 

with  crimson  pigment  on  the  fins,  or  blue  pigment  on  the 
back,  or  jet-black  pigment  all  over  the  head,  or  with  varied 
combinations  of  all  these.  Their  instinct  is  to  display  all 
these  to  the  best  advantage,  even  though  the  conspicuous 
hues  lead  to  their  own  destruction.  Against  this  contin- 
gency Nature  provides  a  superfluity  of  males. 

Among  the  birds  the  male  in  spring  is  in  very  many 
species  provided  with  an  ornamental  plumage  which  he 
sheds  when  the  breeding  season  is  over.  The  scarlet,  crim- 
son, orange,  blue,  black,  and  lustrous  colors  of  birds  are 
commonly  seen  only  on  the  males  in  the  breeding  season, 
the  young  males  and  all  males  in  the  fall  having  the  plain 
brown  gray  or  streaky  colors  of  the  female.  Among  the 
singing  birds  it  is  chiefly  the  male  that  sings,  and  his  voice 
and  the  instinct  to  use  it  are  commonly  lost  when  the  young 
are  hatched  in  the  nest. 

Among  polygamous  mammals  the  male  is  usually  much 
larger  than  the  female,  and  his  courtship  is  often  a 
struggle  with  other  males  for  the  possession  of  the  female. 
Among  the  deer  the  male,  armed  with  great  horns,  fight 
to  the  death  for  the  possession  of  the  female  or  for  the 
mastery  of  the  herd.  The  fur-seal  has  on  an  average  a 
family  of  about  thirty-two  females  (Fig.  71),  and  for  the 
control  of  his  harem  others  are  ready  at  all  times  to  dispute 
the  possession.  But  with  monogamous  animals  like  the 
true  or  hair  seal  or  the  fox,  where  a  male  mates  with  a 
single  female,  there  is  no  such  discrepancy  in  size  and 
strength,  and  the  warlike  force  of  the  male  is  spent  on  out- 
side enemies,  not  on  his  own  species. 

137.  Reproduction. — The  movements  of  many  migra- 
tory animals  are  mainly  controlled  by  the  impulse  to  repro- 
duce. Some  pelagic  fishes,  especially  flying-fishes  and  fishes 
allied  to  the  mackerel,  swim  long  distances  to  a  region 
favorable  for  a  deposition  of  spawn.  Some  species  are 
known  only  in  the  waters  they  make  their  breeding  homes, 
the  individuals  being  scattered  through  the  wide  seas  at 


250  ANIMAL  LIFE 

other  times.  Many  fresh-water  fishes,  as  trout,  suckers, 
etc.,  forsake  the  large  streams  in  the  spring,  ascending  the 
small  brooks  where  they  can  rear  their  young  in  greater 
safety.  Still  others,  known  as  anadromous  fishes,  feed 
and  mature  in  the  sea,  but  ascend  the  rivers  as  the  im- 
pulse of  reproduction  grows  strong.  Among  such  species 
are  the  salmon,  shad,  alewife,  sturgeon,  and  striped  bass  in 
American  waters.  The  most  noteworthy  case  of  the  ana- 
dromous instinct  is  found  in  the  king  salmon  or  quinnat 
of  the  Pacific  coast.  This  great  fish  spawns  in  November. 
In  the  Columbia  Eiver  it  begins  running  in  March  and 
April,  spending  the  whole  summer  in  the  ascent  of  the 
river  without  feeding.  By  autumn  the  individuals  are 
greatly  changed  in  appearance,  discolored,  worn,  and  distort- 
ed. On  reaching  the  spawning  beds,  some  of  them  a  thou- 
sand miles  from  the  sea,  the  female  deposits  her  eggs  in 
the  gravel  of  some  shallow  brook.  After  they  arc  fertilized 
both  male  and  female  drift  tail  foremost  and  helpless  down 
the  stream,  none  of  them  ever  surviving  to  reach  the  sea. 
The  same  habits  are  found  in  other  species  of  salmon  of 
the  Pacific,  but  in  most  cases  the  individuals  of  other  spe- 
cies do  not  start  so  early  or  run  so  far.  A  few  species  of 
fishes,  as  the  eel,  reverse  this  order,  feeding  in  the  rivers 
and  brackish  creeks,  dropping  down  to  the  sea  to  spawn. 

The  migration  of  birds  has  relation  to  reproduction  as 
well  as  to  changes  of  weather.  As  soon  as  they  reach  their 
summer  homes,  courtship,  mating,  nest-building,  and  the 
care  of  the  young  occupy  the  attention  of  every  species. 

138.  Care  of  the  young. — In  the  animal  kingdom  one  of 
the  great  factors  in  development  has  been  the  care  of  the 
young.  This  feature  is  a  prominent  one  in  the  specializa- 
tion of  birds  and  mammals.  When  the  young  are  cared  for 
the  percentage  of  loss  in  the  struggle  for  life  is  greatly  re- 
duced, the  number  of  births  necessary  to  the  maintenance 
of  the  species  is  much  less,  and  the  opportunities  for  spe- 
cialization in  other  relations  of  life  are  much  greater. 


INSTINCT  AND  REASON  251 

In  these  regards,  the  nest-building  and  home-making 
animals  have  the  advantage  over  those  that  have  not  these 
instincts.  The  animals  that  mate  for  life  have  the  advan- 
tage over  polygamous  animals,  and  those  whose  social  or 
mating  habits  give  rise  to  a  division  of  labor  over  those 
with  instincts  less  highly  specialized. 

The  interesting  instincts  and  habits  connected  with  nest 
or  home  building  and  the  care  of  the  young  are  discussed 
in  the  next  chapter. 

139.  Variability  of  instincts. — When  we  study  instincts 
of  animals  with  care  and  in  detail,  we  find  that  their  regu- 
larity is  much  less  than  has  been  supposed.     There  is  as 
much  variation  in  regard  to  instinct  among  individuals  as 
there  is  with  regard  to  other  characters  of  the  species.' 
Some  power  of  choice  is  found  in  almost  every  operation  of 
instinct.     Even  the  most  machine-like  instinct  shows  some 
degree  of  adaptability  to  new  conditions.     On  the  other 
hand,  in  no  animal  does  reason  show  entire  freedom  from 
automatism  or  reflex  action.     "  The  fundamental  identity 
of  instinct  with  intelligence,"  says  an  able  investigator,  "  is 
shown  in  their  dependence  upon  the  same  structural  mech- 
anism (the  brain  and  nerves)  and  in  their  responsive  adap- 
tability." 

140.  Reason. — Reason  or  intellect,  as  distinguished  from 
instinct,  is  the  choice,  more  or  less  conscious,  among  re- 
sponses to  external  impressions.     Its  basis,  like  that  of  in- 
stinct, is  in  reflex  action.     Its  operations,  often  repeated, 
become  similarly  reflex  by  repetition,  and  are  known  as 
habit.     A  habit  is  a  voluntary  action  repeated  until  it  be- 
comes reflex.     It  is  essentially  like  instinct  in  all  its  mani- 
festations.    The  only  evident  difference  is  in  its  origin. 
Instinct  is  inherited.     Habit  is  the  reaction  produced  with- 
in the  individual  by  its  own  repeated    actions.      In  the 
varied  relations  of  life  the  pure  reflex  action  becomes  inade- 
quate.    The  sensorium  is  offered  a  choice  of  responses.    To 
choose  one  and  to  reject  the  others  is  the  function  of  intel- 


252  ANIMAL  LIFE 

lect  or  reason.  While  its  excessive  development  in  man 
obscures  its  close  relation  to  instinct,  both  shade  off  by 
degrees  into  reflex  action.  Indeed,  no  sharp  line  can  be 
drawn  between  unconscious  and  subconscious  choice  of 
reaction  and  ordinary  intellectual  processes. 

Most  animals  have  little  self-consciousness,  and  their 
reasoning  powers  at  best  are  of  a  low  order ;  but  in  kind, 
at  least,  the  powers  are  not  different  from  reason  in  man. 
A  horse  reaches  over  the  fence  to  be  company  to  another. 
This  is  instinct.  When  it  lets  down  the  bars  with  its  teeth, 
that  is  reason.  When  a  dog  finds  its  way  home  at  night  by 
the  sense  of  smell,  this  may  be  instinct ;  when  he  drags  a 
stranger  to  his  wounded  master,  that  is  reason.  When  a 
jack-rabbit  leaps  over  the  brush  to  escape  a  dog,  or  runs  in 
a  circle  before  a  coyote,  or  when  it  lies  flat  in  the  grass  as  a 
round  ball  of  gray  indistinguishable  from  grass,  this  is  in- 
stinct. But  the  same  animal  is  capable  of  reason — that  is, 
of  a  distinct  choice  among  lines  of  action.  Not  long  ago  a 
rabbit  came  bounding  across  the  university  campus  at  Palo 
Alto.  As  it  passed  a  corner  it  suddenly  faced  two  hunting 
dogs  running  side  by  side  toward  it.  It  had  the  choice  of 
turning  back,  its  first  instinct,  but  a  dangerous  one ;  of 
leaping  over  the  dogs,  or  of  lying  flat  on  the  ground.  It 
chose  none  of  these,  and  its  choice  was  instantaneous.  It 
ceased  leaping,  ran  low,  and  went  between  the  dogs  just  as 
they  were  in  the  act  of  seizing  it,  and  the  surprise  of  the 
dogs,  as  they  stopped  and  tried  to  hurry  around,  was  the 
same  feeling  that  a  man  would  have  in  like  circumstances. 

On  the  open  plains  of  Merced  County,  California,  the 
jack-rabbit  is  the  prey  of  the  bald  eagle.  Not  long  since  a 
rabbit  pursued  by  an  eagle  was  seen  to  run  among  the 
cattle.  Leaping  from  cow  to  cow,  he  used  these  animals 
as  a  shelter  from  the  savage  bird.  When  the  pursuit  was 
closer,  the  rabbit  broke  cover  for  a  barbed  wire  fence. 
When  the  eagle  swooped  down  on  it,  the  rabbit  moved  a 
few  inches  to  the  right,  and  the  eagle  could  not  reach  him 


INSTINCT  AND  REASON  253 

through  the  fence.  When  the  eagle  came  down  on  the 
other  side,  he  moved  across  to  the  first.  And  this  was  con- 
tinued until  the  eagle  gave  up  the  chase.  It  is  instinct 
that  leads  the  eagle  to  swoop  on  the  rabbit.  It  is  instinct 
again  for  the  rabbit  to  run  away.  But  to  run  along  the  line 
of  a  barbed  wire  fence  demands  some  degree  of  reason.  If 
the  need  to  repeat  it  arose  often  in  the  lifetime  of  a  single 
rabbit  it  would  become  a  habit. 

The  difference  between  intellect  and  instinct  in  lower 
animals  may  be  illustrated  by  the  conduct  of  certain  mon- 
keys brought  into  relation  with  new  experiences.  At  one 
time  we  had  two  adult  monkeys,  "  Bob  "  and  "  Jocko,"  be- 
longing to  the  genus  Macacus.  Neither  of  these  possessed 
the  egg-eating  instinct.  At  the  same  time  we  had  a  baby 
monkey,  "  Mono,"  of  the  genus  Cercopithecus.  Mono  had 
never  seen  an  egg,  but  his  inherited  impulses  bore  a  direct 
relation  to  feeding  on  eggs,  just  as  the  heredity  of  Macacus 
taught  the  others  how  to  crack  nuts  or  to  peel  fruit. 

To  each  of  these  monkeys  we  gave  an  egg,  the  first  that 
any  of  them  had  ever  seen.  The  baby  monkey,  Mono, 
being  of  an  egg-eating  race,  devoured  his  egg  by  the  opera- 
tion of  instinct  or  inherited  habit.  On  being  given  the 
egg  for  the  first  time,  he  cracked  it  against  his  upper  teeth, 
making  a  hole  in  it,  and  sucked  out  all  the  substance. 
Then  holding  the  egg-shell  up  to  the  light  and  seeing  that 
there  was  no  longer  anything  in  it,  he  threw  it  away.  All 
this  he  did  mechanically,  automatically,  and  it  was  just  as 
well  done  with  the  first  egg  he  ever  saw  as  with  any  other 
he  ate.  All  eggs  since  offered  him  he  has  treated  in  the 
same  way. 

The  monkey  Bob  took  the  egg  for  some  kind  of  nut. 
He  broke  it  against  his  upper  teeth  and  tried  to  pull  off 
the  shell,  when  the  inside  ran  out  and  fell  on  the  ground. 
He  looked  at  it  for  a  moment  in  bewilderment,  took  both 
hands  and  scooped  up  the  yolk  and  the  sand  with  which  it 
was  mixed  and  swallowed  the  whole.  Then  he  stuffed  the 


254  ANIMAL   LIFE 

shell  itself  into  his  mouth.  This  act  was  not  instinctive. 
It  was  the  work  of  pure  reason.  Evidently  his  race  was 
not  familiar  with  the  use  of  eggs  and  had  acquired  no  in- 
stincts regarding  them.  He  would  do  it  better  next  time. 
Eeason  is  an  inefficient  agent  at  first,  a  weak  tool;  but 
when  it  is  trained  it  becomes  an  agent  more  valuable  and 
more  powerful  than  any  instinct. 

The  monkey  Jocko  tried  to  eat  the  egg  offered  him  in 
much  the  same  way  that  Bob  did,  but,  not  liking  the  taste, 
he  threw  it  away. 

The  confusion  of  highly  perfected  instinct  with  intellect 
is  very  common  in  popular  discussions.  Instinct  grows 
weak  and  less  accurate  in  its  automatic  obedience  as  the 
intellect  becomes  available  in  its  place.  Both  intellect  and 
instinct  are  outgrowths  from  the  simple  reflex  response  to 
external  conditions.  But  instinct  insures  a  single  definite 
response  to  the  corresponding  stimulus.  The  intellect  has 
a  choice  of  responses.  In  its  lower  stages  it  is  vacillating 
and  ineffective ;  but  as  its  development  goes  on  it  becomes 
alert  and  adequate  to  the  varied  conditions  of  life.  It 
grows  with  the  need  for  improvement.  It  will  therefore 
become  impossible  for  the  complexity  of  life  to  outgrow 
the  adequacy  of  man  to  adapt  himself  to  its  conditions. 

Many  animals  currently  believed  to  be  of  high  intelli- 
gence are  not  so.  The  fur-seal,  for  example,  finds  it  way 
back  from  the  long  swim  of  two  or  three  thousand  miles 
through  a  foggy  and  stormy  sea,  and  is  never  too  late  or  too 
early  in  arrival.  The  female  fur-seal  goes  two  hundred 
miles  to  her  feeding  grounds  in  summer,  leaving  the  pup 
on  the  shore.  After  a  week  or  two  she  returns  to  find  him 
within  a  few  rods  of  the  rocks  where  she  had  left  him. 
Both  mother  and  young  know  each  other  by  call  and  by 
odor,  and  neither  is  ever  mistaken,  though  ten  thousand 
other  pups  and  other  mothers  occupy  the  same  rookery. 
But  this  is  not  intelligence.  It  is  simply  instinct,  because 
it  has  no  element  of  choice  in  it.  Whatever  its  ancestors 


INSTINCT  AND  EEASON  255 

were  forced  to  do  the  fur-seal  does  to  perfection.  Its  in- 
stincts are  perfect  as  clockwork,  and  the  necessities  of 
migration  must  keep  them  so.  But  if  brought  into  new 
conditions  it  is  dazed  and  stupid.  It  can  not  choose  when 
different  lines  of  action  are  presented. 

The  Bering  Sea  Commission  once  made  an  experiment 
on  the  possibility  of  separating  the  young  male  fur-seals, 
or  "  killables,"  from  the  old  ones  in  the  same  band.  The 
method  was  to  drive  them  through  a  wooden  chute  or  run- 
way with  two  valve-like  doors  at  the  end.  These  animals  can 
be  driven  like  sheep,  but  to  sort  them  in  the  way  proposed 
proved  impossible.  The  most  experienced  males  would 
beat  their  noses  against  a  closed  door,  if  they  had  seen  a 
seal  before  them  pass  through  it.  That  this  door  had  been 
shut  and  another  opened  beside  it  passed  their  comprehen- 
sion. They  could  not  choose  the  new  direction.  In  like 
manner  a  male  fur-seal  will  watch  the  killing  and  skinning 
of  his  mates  with  perfect  composure.  He  will  sniff  at  their 
blood  with  languid  curiosity  ;  so  long  as  it  is  not  his  own 
it  does  not  matter.  That  his  own  blood  may  flow  out  on 
the  ground  in  a  minute  or  two  he  can  not  foresee. 

Eeason  arises  from  the  necessity  for  a  choice  among  ac- 
tions. It  may  arise  as  a  clash  among  instincts  which  forces 
on  the  animal  the  necessity  of  choosing.  A  doe,  for  ex- 
ample, in  a  rich  pasture  has  the  instinct  to  feed.  It  hears 
the  hounds  and  has  the  instinct  to  flee.  Its  fawn  may  be 
with  her  and  it  is  her  instinct  to  remain  and  protect  it. 
This  may  be  done  in  one  of  several  ways.  In  proportion  as 
the  mother  chooses  wisely  will  be  the  fawn's  chance  of  sur- 
vival. Thus  under  difficult  conditions,  reason  or  choice 
among  actions  rises  to  the  aid  of  the  lower  animals  as  well 
as  man. 

141.  Mind. — The  word  mind  is  popularly  used  in  two 
different  senses.  In  the  biological  sense  the  mind  is  the 
collective  name  for  the  functions  of  the  sensorium  in  men 
and  animals.  It  is  the  sum  total  of  all  psychic  changes, 


256  ANIMAL  LIFE 

actions  and  reactions.  Under  the  head  of  psychic  functions 
are  included  all  operations  of  the  nervous  system  as  well  as 
all  functions  of  like  nature  which  may  exist  in  organisms 
without  specialized  nerve  fibers  or  nerve  cells.  As  thus  de- 
fined mind  would  include  all  phenomena  of  irritability,  and 
even  plants  have  the  rudiments  of  it.  The  operations  of 
the  mind  in  this  sense  need  not  be  conscious.  With  the 
lower  animals  almost  all  of  them  are  automatic  and  uncon- 
scious. With  man  most  of  them  must  be  so.  All  func- 
tions of  the  sensorium,  irritability,  reflex  action,  instinct, 
reason,  volition,  are  alike  in  essential  nature  though  differ- 
ing greatly  in  their  degree  of  specialization. 

In  another  sense  the  term  mind  is  applied  only  to  con- 
scious reasoning  or  conscious  volition.  In  this  sense  it  is 
mainly  an  attribute  of  man,  the  lower  animals  showing  it 
in  but  slight  degree.  The  discussion  as  to  whether  lower 
animals  have  minds  turns  on  the  definition  of  mind,  and 
our  answer  to  it  depends  on  the  definition  we  adopt. 


Pro.  155. — A  "pointer"  dog  in  the  act  of  "pointing,"  a  specialized  instinct. 
(Permission  of  G.  O.  Shields,  publisher  of  Recreation.) 


CHAPTER  XV 

HOMES    AND    DOMESTIC    HABITS 

142.  Importance  of  care  of  the  young. — The  nest-building 
and  domestic  habits  of  animals  are  adaptations,  but  adapta- 
tions of  behavior  or  habit  rather  than  of  structure,  and  are 
based  on  instinct,  intelligence,  and  reason.    These  instincts 
and  habits  are  among  the  most  important  shown  by  animals, 
because  on  them  depends  largely  the  continuance  of  the 
species.     Of  primary  importance  in  the  perpetuation  of  the 
species  is  the  possession  by  animals  of  adaptations  of  struc- 
ture and  behavior,  which  help  the  individual  live  long  enough 
to  attain  full  development  and  to  leave  offspring.     But  in 
the  case  of  many  animals  a  successful  start  in  life  on  the 
part  of  the  offspring  depends  on  the  existence  in  the  par- 
ents of  certain  domestic  habits  concerned  with  the  care  and 
protection  of  their  young.    The  young  of  many  animals  de- 
pend absolutely,  for  a  part  of  their  lifetime,  on  this  parental 
care.    In  these  cases  it  is  quite  as  necessary  for  the  continued 
existence  of  the  species  that  the  habits  that  afford  this  care 
be  successful  as  that  the  parent  should  come  successfully  to 
mature  development  and  to  the  production  of  offspring. 

143.  Care  of  the  young,  and  communal  life. — The  nest- 
building  or  home-making  habits  and  the  continued  per- 
sonal care  of  the  young  for  a  part  of  their  lifetime  are  most 
highly  developed  and  widespread  among  the  birds,  mam- 
mals, and  insects ;  and  it  is  both  among  the  insects  and 
the  higher  vertebrates  that  we  find  most  developed  those 
social  and  communal  habits  discussed  in  Chapter  IX.     The 
principal  activities  of  an  animal  community  have  to  do 

18  257 


258  ANIMAL  LIFE 

with  the  protection  and  sustenance  of  the  young,  and  the 
care  of  the  young  is  undoubtedly  a  chief  factor  in  the  de- 
velopment of  communal  life. 

144.  The  invertebrates  (except  spiders  and  insects).— 
Among  the  lower  invertebrates  parental  aid  to  the  young  is 
confined  almost  exclusively  to  exhibitions  of  pure  instinct. 
And  this  is  true  of  many  of  the  higher  animals  also.  Eggs 
are  deposited  in  sheltered  places,  and  in  such  places  and 
under  such  circumstances  that  the  young  on  hatching  will 
find  themselves  close  to  a  supply  of  their  natural  food.  The 
depositing  of  eggs  in  water  by  parents  with  terrestrial  hab- 
its whose  young  are  aquatic,  is  an  example.  The  toad, 
which  lives  on  land,  feeding  on  insects,  has  young  which 
live  in  water  and  feed  on  minute  aquatic  plants  (algae). 
The  dragon  fly,  that  hawks  over  the  pond  or  brook  with 
glistening  wings,  has  young  that  crawl  in  the  slime  and 
mud  at  the  bottom  of  the  pool.  With  most  animals,  after 
laying  eggs,  the  parents  show  no  further  solicitude  toward 
their  offspring.  The  eggs  are  left  to  the  vicissitudes  of 
fortune,  and  the  parents  know  nothing  of  their  fate.  In 
many  cases  the  parent  dies  before  the  young  are  hatched. 
The  mammals  and  birds  are  the  only  two  great  groups  ex- 
cepted,  although  there  are  numerous  specific  exceptions 
scattered  among  the  lower  invertebrates,  fishes,  batrachians, 
and  higher  invertebrates,  notably  the  insects. 

There  are  no  instances  of  care  of  the  young  after  hatch- 
ing among  the  sponges,  polyps,  worms,  or  star-fishes  and 
sea-urchins,  and  but  few  among  the  crustaceans  and  mol- 
lusks.  But  there  are  in  some  of  these  groups  a  few  cases 
of  nest  or  home  building  in  a  crude  and  simple  way.  Cer- 
tain sea-urchins  (Fig.  156)  and  worms  and  mollusks  bore 
into  stones,  and  remain  in  the  shelter  afforded  by  the  cav- 
ity. A  shell-fish  (Lima  hiams)  cements  together  bits  of 
coralline,  shells,  and  sand  to  form  a  crude  nest  or  hiding- 
place.  The  cray-fish  digs  a  cylindrical  burrow  in  the  ground 
in  which  it  can  hide. 


HOMES  AND  DOMESTIC  HABITS 


259 


145.  Spiders.— Most  spiders  spin  silken  cocoons  or  sacs 
in  which  to  deposit  their  eggs.  Some  spiders  carry  this 
egg-filled  cocoon  about  with  them  for  the  sake  of  protect- 
ing the  eggs.  After  hatching,  the  spiderlings  remain  in  the 
cocoon  a  short  time,  feeding  on  each  other !  Thus  only  the 


FIG.  156.— Sea-nrchins  living  in  holes  bored  into  rocks  of  the  seashore  below  high- 
tide  line. 

strongest  survive  and  issue  from  the  cocoon  to  earn  their 
living  in  the  outer  world.  "With  certain  species  of  spiders 
the  young  after  hatching  leave  the  cocoon  and  gather  on 
the  back  of  the  mother  and  are  carried  about  by  her  for 
some  time.  In  connection  with  their  webs  or  snares  many 
spiders  have  silken  tunnels  or  tubes  in  which  to  lie  hidden, 
a  sort  of  sheltering  nest.  Those  spiders  that  live  on  the 
ground  make  for  themselves  cylindrical  burrows  or  holes 
in  the  ground,  usually  lined  with  silk,  in  which  they  hide 
when  not  hunting  for  food.  Especially  interesting  among 
the  many  kinds  of  these  spider  nests  are  the  burrows  of 
the  various  trap-door  spiders.  These  spiders  are  common 
in  California  and  some  other  far  Western  States.  The  bur- 


260  ANIMAL  LIFE 

row  (Fig.  157)  or  cylindrical  hole  is  closed  above  by  a  silken, 
thick,  hinged  lid  or  door,  a  little  larger  in  diameter  than 
the  hole  and  neatly  beveled  on  the  edge,  so  as  to  fit  tightly 
into  and  perfectly  cover  the  hole  when  closed.  The  upper 
surface  of  the  door  is  covered  with  soil,  bits  of  leaves,  and 
wood,  so  as  to  resemble  very  exactly  the  ground  surface 
about  it.  "We  have  found  these  trap-door  nests  in  Cali- 
fornia in  moss-covered  ground,  and  here  the  lids  of  the  nests 
were  always  covered  with  green,  growing  moss. 

An  English  naturalist  who  studied  the  habits  of  these 
trap-door  spiders  found  that  if  he  removed  the  soil  and  bits 
of  bark  and  twigs,  or  the  moss,  from  the  upper  surface  of 
the  lid  the  spider  always  re-covered  it.  It  is,  of  course, 
plain  that  by  means  of  this  covering  the  nest  is  perfectly 
concealed,  the  surface  of  the  closed  door  not  being  dif- 
ferent from  the  surrounding  ground  surface.  This  natu- 
ralist finally  removed  the"  moss  not  only  from  the  surface 
of  a  trap-door,  but  also  from  all  the  ground  in  a  circle  of  a 
few  feet  about  the  nest.  The  next  day  he  found  that  the 
spider  had  brought  moss  from  outside  the  cleared  space 
and  covered  the  trap-door  with  it,  thus  making  it  very  con- 
spicuous in  the  cleared  ground  space.  The  spider's  instinct 
was  not  capable  of  that  quick  modification  to  allow  it  to  do 
what  a  reasoning  animal  would  have  done — namely,  cov- 
ered the  trap-door  only  with  soil  to  make  it  resemble  the 
cleared  ground  about  it. 

Another  interesting  nest-making  spider  is  the  turret- 
spider,  that  builds  up  a  little  tower  (Fig.  158)  of  sticks  and 
soil  and  moss  above  its  burrow.  The  sticks  of  which  this 
burrow  are  built  are  an  inch  or  two  in  length,  and  are 
arranged  in  such  manner  as  make  the  turret  five-sided. 
The  sticks  are  fastened  together  with  silk,  and  the  turret 
is  made  two  or  three  inches  high.  This  turret-building 
spider  is  one  of  those  that  carry  about  their  egg-cocoon 
with  them.  A  female  of  this  spider  in  captivity  was  ob- 
served to  pay  much  attention  to  caring  for  this  cocoon. 


262 


ANIMAL  LIFE 


"  If  the  weather  was  cold  or  damp,  she  retired  to  her  tunnel ; 
but  if  the  jar  in  which  she  lived  was  set  where  the  sun 

could  shine  upon  it,  she  soon  re- 
appeared and  allowed  the  cocoon 
to  bask  in  the  sunlight.     If  the 
jar  was  placed  near  a  stove  that 
had  a  fire  in  it,  the  cocoon  was 
put  on  the  side  next  the  source 
of    warmth ;   if   the    jar 
was  turned  around,  she 
lost  no  time   in  moving 
the  cocoon  to  the  warmer 
side.     Two  months  after 
the   eggs   were  laid  the 
young  spiders  made  their 
appearance  and  immediately 
perched  upon  their  mother,  many 
on  her  back,  some  on  her  head, 
and  even  on  her  legs.     She  car- 
ried them  about  with  her  and  fed 
them,  and  until  they  were  older 
they  never  left  their  mother  for 
a  moment." 

146.  Insects. — So  much  space 
has  already  been  devoted  to  an 
account  of  the  elaborate  nest-making  and  domestic  habits 
of  the  bees,  ants,  and  termites  (see  Chapter  IX),  that  we 
need  in  this  place  merely  refer  to  that  account.  It  is 
among  these  social  insects  that  the  most  interesting  and 
highly  specialized  habits  connected  with  the  care  of  the 
young  and  the  building  of  homes  are  found. 

Many  insects  make  for  themselves  simple  burrows  or 
nests  in  the  ground  or  in  wood.  The  young  or  larvae  of 
certain  moths  burrow  about  in  the  soft  inside  tissue  of 
leaves,  and  the  whole  life  of  the  moth  except  its  short  adult 
stage  is  passed  inside  the  leaf.  These  larvae  are  called  leaf- 


FIG.  158.— Nest  of  the  turret- 
spider. 


HOMES  AND  DOMESTIC  HABITS  263 

miners.  The  larvae  of  some  moths  and  of  many  hymenop- 
terous  insects  live  in  galls  on  live  plants.  These  galls  are 
simply  abnormal  growths  of  plant  tissue,  and  are  caused  hy 
the  irritating  effect  on  the  tissue  of  the  larvae  which  hatch 
from  eggs  that  have  been  thrust  into  the  soft  plant  sub- 
stance by  the  female  insects.  In  the  familiar  galls  on  the 
golden-rod  live  the  larvae  of  a  small  moth,  and  in  the  vari- 
ous kinds  of  oak  galls  live  the  young  of  the  numerous  spe- 
cies of  Cynipiclce,  the  hymenopterous  gall  insects.  The  tiny 
larvae  of  some  of  the  midges  live  in  small  galls  on  various 
plants.  To  this  last  group  of  gall-making  insects  belongs 
the  dreaded  Hessian  fly,  the  most  destructive  insect  pest  of 
wheat. 

Among  the  bees  and  wasps  only  a  few  species,  compara- 
tively, are  communal  or  live  in  communities.  But  nearly 
all  the  wasps  and  bees,  whether  social  or  solitary  in  habit, 
build  nests  for  their  young  and  provide  the  young  with 
food,  either  by  storing  it  in  the  nest  or  by  hunting  for  it 
and  bringing  it  to  the  nest  as  long  as  the  young  are  in  the 
larval  condition.  The  "mud-daubers"  or  thread-waisted 
wasps  make  nests  of  mud  attached  to  the  lower  surface  of 
flat  stones,  to  the  ceiling  of  buildings,  or  in  other  out-of- 
the-way  and  safe  places.  These  nests  usually  have  the  form 
of  several  tubes  an  inch  or  so  long  placed  side  by  side.  In 
each  of  the  tubes  or  cells  an  egg  is  laid,  and  with  it  a 
spider  which  has  been  stung  so  as  to  bo  paralyzed  but 
not  killed.  When  the  young  wasp  hatches  from  the  egg 
as  a  grub  or  larva,  it  feeds  on  the  unfortunate  spider. 
Others  of  the  solitary  wasps  make  nests  in  the  ground 
or  in  wood,  and  all  of  them  provision  their  nests  with 
some  particular  kind  of  insect  or  spider.  Some  use  only 
caterpillars,  some  plant-lice,  and  some  grasshoppers.  Simi- 
larly the  solitary  bees  make  nests  in  the  ground  as  do  the 
mining-bees,  or  in  wood  as  do  the  carpenter-bees,  or  by 
cutting  and  fastening  together  leaves,  as  do  the  leaf-cutting 
bees.  The  bees  provision  their  nests,  not  with  paralyzed 


264  ANIMAL  LIFE" 

insects,  but  with  masses  of  pollen  or  pollen  mixed  with 
nectar. 

147.  The  vertebrates. — It  is  among  the  vertebrates,  espe- 
cially in  the  higher  groups,  the  birds  and  mammals,  that 
we  find  the  care  of  the  young  most  perfectly  undertaken 
and  most  widespread. 

Among  the  fishes,  the  lowest  of  the  vertebrates,  most 
species  content  themselves  with  the  laying  of  many  eggs  in 
a  situation  best  suited  for  their  safe  hatching.  But  some 
species  show  interesting  domestic  habits.  The  female  cat- 
fish swims  about  with  her  brood,  much  as  a  hen  moves 
about  with  her  chickens.  Some  of  the  larger  ocean  cat- 
fish of  the  tropics  receive  the  eggs  or  the  young  within  the 
mouth  for  safety  jyn  time  of  danger.  Certain  sunfishes  care 
for  their  young,  keeping  them  together  in  still  places  in  the 
brook.  They  also  make  some  traces  of  a  nest,  which  the 
male  defends.  The  male  salmon  scoops  out  gravel  to  make 
a  shallow  nest,  in  which  the  female  deposits  her  eggs.  The 
male  then  covers  the  eggs.  The  males  of  the  species  of 
pipe-fish  and  sea-horses  receive  the  eggs  of  the  female  into 
a  groove  or  sac  between  the  folds  of  skin  on  the  lower  part 
of  the  tail.  Here  they  are  kept  until  the  little  fishes  are 
large  enough  to  swim  about  for  themselves.  The  brave 
little  sticklebacks  build  tiny  nests  about  an  inch  and  a  half 
or  two  inches  in  diameter,  with  a  small  opening  at  the  top. 
The  eggs  are  laid  in  this  nest,  and  the  young  fish  remain  in 
it  some  time  after  hatching.  The  male  parent  jealously 
guards  the  nest,  and  fights  bravely  with  would-be  intruders. 

The  batrachians  and  reptiles  rarely  show  any  care  for 
their  young.  The  eggs  of  most  batrachians  are  laid  in  the 
water  and  left  by  the  female.  The  males  of  the  Surinam 
toad  receive  the  eggs  in  pits  of  the  spongy  skin  of  the  back, 
where  they  remain  until  the  young  hatch.  The  eggs  of 
snakes  are  laid  under  logs  or  buried  in  the  sand,  and  no 
further  attention  is  given  them  by  the  parents. 

Among  the  birds,  on  the  other  hand,  nest-building  and 


HOMES  AND  DOMESTIC  HABITS 


265 


care  of  the  young  are  the  rule,  and  a  high  degree  of  devel- 
opment in  these  habits  is  reached.  All  of  us  are  familiar 
with  many  different  kinds  of  nests,  from  the  few  twigs 
loosely  put  together  hy  the  mourning-dove  to  the  firm, 
closely  knit,  wool  or  feather  lined  nest  of  the  humming- 
bird (Fig.  159),  and  the  basket-like  hanging  nest  of  the 


FIG.  159.— Nest  and  eggs  of  the  Rufus  humming-bird  (Trochilus  rufus).    Photograph 
by  J.  O.  SNYDBB,  Stanford  University,  California. 


2C6 


ANIMAL  LIFE 


oriole  (Fig.  161).  Not  all  birds  make  nests.  On  the  rocky 
islets  of  the  northern  oceans,  where  thousands  of  puffins 
and  auks  and  other  maritime  birds  gather  to  breed,  the 
eggs  are  laid  on  the  bare  rock.  At  the  other  extreme  is 
the  tailor  bird  of  India,  which  sews  together  leaves  by 
means  of  fibrous  strips  plucked  from  a  growing  plant  to 


FIG.  160.— Neet  and  young  of  the  Rufus  humming-bird  ( Trochilus  rufus).  Photograph 
bjr  J.  O.  SNTDEB,  Stanford  University,  California. 


HOMES  AND  DOMESTIC  HABITS 


267 


FIG.  161. Baltimore  orioles  and  nest ;  the  male  in  upper  left-hand  corner  of  figure. 

form  a  long,  bag-like  nest  (Fig.  162).  In  the  degree  of 
care  given  the  nestlings  there  is  also  much  difference.  The 
robin  brings  food  to  the  helpless  young  for  many  days,  and 


268 


ANIMAL  LIFE 


finally  teaches  it  to  fly  and  to  hunt  for  food  for  itself. 
Young  chickens  are  not  so  helpless  as  the  nestling  robins, 
but  are  able  to  run  about,  and  under  the  guiding 

care   of    the   hen  mother  to        /^      pick  up  food  for 

themselves. 

Among  the  mam- 
mals the  young  are 
always  given  some 
degree  of  care.  Ex- 
cepting in  the  case 
of  the  duck-bills,  the 
lowest  of  the  mam- 
mals, the  young  are 
born  alive — that  is, 
are  not  hatched  from 
eggs  laid  outside  the 
body— and  are  nour- 
ished after  birth  for 
a  shorter  or  longer 
time  with  milk 
drawn  from  the 
body  of  the  mother. 
Before  birth  the 
young  undergoes  a 
longer  or  shorter 

period  of  development  and  growth  in  the  body  of  the 
mother,  being  nourished  by  the  blood  of  the  mother.  The 
nests  or  homes  of  mammals  present  varying  degrees  of 
elaborateness,  from  a  simple  cave-like  hole  in  the  rocks 
or  ground  to  the  elaborately  constructed  villages  of  the 
beavers  with  their  dams  and  conical  several-storied  houses 
(Fig.  163).  The  wood-rat  piles  together  sticks  and  twigs 
in  what  seems,  from  the  outside,  a  most  haphazard  fashion, 
but  which  results  in  the  construction  of  a  convenient  and 
ingenious  nest.  The  moles  and  pocket-gophers  (Fig.  165) 
build  underground  nests  composed  of  chambers  and  gal- 


FIG.  162.— Tailor-bird  (OrnUhotomus  sutorius) 
and  nest. 


2TO 


ANIMAL  LIFE 


FIG.  164.— Nest  of  the  Californian  bnsh-tit  (Psaltriparus  minimus).    Photograph  by 
G.  O.  SNYDEK,  Stanford  University,  California. 

leries.      The  prairie-dogs  make  burrows  in  groups,  forming 
large  villages. 

The  devotion  to  their  young  displayed  by  birds   and 
mammals  is  familiar  to  us.     The  parents  will  often  risk  or 


HOMES  AND  DOMESTIC  HABITS 


271 


suffer  the  loss  of  their  own  lives  in  protecting  their  off 
spring  from  enemies.  Many  mother  birds  have  the  instinct 
to  flutter  about  a  discovered  nest  crying  and  apparently 
broken-winged,  thus  leading  the  predatory  fox  or  weasel  to 


FIG.  165.— Nest  and  run-way  of  the  pocket-gopher. 

fix  his  attention  on  the  mother  and  to  leave  the  nest  un- 
harmed. This  development  of  parental  care  and  protec- 
tion of  the  young  reaches  its  highest  degree  in  the  case  of 
the  human  species.  The  existence  of  the  family,  which  is 
the  unit  of  human  society,  rests  on  this  high  development 
of  care  for  the  young. 


CHAPTEE  XVI 

GEOGRAPHICAL   DISTRIBUTION   OF   ANIMALS 

148.  Geographical  distribution.— Under  the  head  of  dis- 
tribution we  consider  the  facts  of  the  diffusion  of  organ- 
isms over  the  surface  of  the  earth,  and  the  laws  by  which 
this  diffusion  is  governed, 

The  geographical  distribution  of  animals  is  often  known 
as  zoogeography.  In  physical  geography  we  may  prepare 
maps  of  the  earth  which  shall  bring  into  prominence  the 
physical  features  of  its  surface.  Such  maps  would  show 
here  a  sea,  here  a  plateau,  here  a  range  of  mountains, 
there  a  desert,  a  prairie,  a  peninsula,  or  an  island.  In  po- 
litical geography  the  maps  show  the  physical  features  of 
the  earth,  as  related  to  the  states  or  powers  which  claim 
the  allegiance  of  the  people.  In  zoogeography  the  realms 
of  the  earth  are  considered  in  relation  to  the  types  or 
species  of  animals  which  inhabit  them.  Thus  a  series  of 
maps  of  the  United  States  could  be  drawn  which  would 
show  the  gradual  disappearance  of  the  buffalo  before  the 
attacks  of  man.  Another  might  be  drawn  which  would 
show  the  present  or  past  distribution  of  the  polar  bear, 
black  bear,  and  grizzly.  Still  another  might  show  the 
original  range  of  the  wild  hares  or  rabbits  of  the  United 
States,  the  white  rabbit  of  the  Northeast,  the  cotton-tail  of 
the  East  and  South,  the  jack-rabbit  of  the  plains,  the  snow- 
shoe  rabbit  of  the  Columbia  Eiver,  the  tall  jack-rabbit  of 
California,  the  black  rabbits  of  the  islands  of  Lower  Cali- 
fonia,  and  the  marsh-hare  of  the  South  and  the  water-hare 
of  the  canebrakes,  and  that  of  all  their  relatives.  Such  a 
272 


FIG.  166.— Map  showing  the  distribution  of  the  clouded  Skipper  butterfly  (Lerema 
accius)  In  the  United  States.  The  butterfly  is  found  in  that  part  of  the  country 
shaded  in  the  map,  a  warm  and  moist  region. — After  SCUDDEK. 


•no 


FIG.  167.— Map  showing  the  distribution  of  the  Canadian  Skipper  butterfly  (Erynnis 
manitoba}  in  the  United  States.  The  butterfly  is  found  in  that  part  of  the 
country  shaded  in  the  map.  This  butterfly  is  subarctic  and  subalpine  in  dis- 
tribution, being  found  only  far  north  or  on  high  mountains,  the  two  southern 
projecting  parts  of  its  range  being  in  the  Rocky  Mountains  and  in  the  Sierra 
Nevada  Mountains.— After  SCUDDEB. 
19 


274  ANIMAL  LIFE 

map  is  very  instructive,  and  it  at  once  raises  a  series  of 
questions  as  to  the  reasons  for  each  of  the  facts  in  geo- 
graphical distribution,  for  it  is  the  duty  of  science  to  sup- 
pose that  none  of  these  facts  is  arbitrary  or  meaningless. 
Each  fact  has  some  good  cause  behind  it. 

149.  Laws  of  distribution. — The  laws  governing  the  dis- 
tribution of  animals  are  reducible  to  three  very  simple 
propositions.     Every  species  of  animal  is  found  in  every  part 
of  the  earth  having  conditions  suitable  for  its  maintenance, 
unless — 

(a)  Its  individuals  have  been  unable  to  reach  this  re- 
gion, through  barriers  of  some  sort ;  or — 

(b)  Having  reached  it,  the  species  is  unable  to  maintain 
itself,  through   lack  of  capacity  for  adaptation,  through 
severity  of  competition  with  other  forms,  or  through  de- 
structive conditions  of  environment ;  or — 

(c)  Having  entered  and  maintained  itself,  it  has  become 
so  altered  in  the  process  of  adaptation  as  to  become  a  spe- 
cies distinct  from  the  original  type. 

150.  Species  debarred  by  barriers. — As  examples  of  the 
first  class  we  may  take  the  absence  of  kingbirds  or  meadow- 
larks  or  coyotes  in  Europe,  the  absence  of  the  lion  and 
tiger  in  South  America,  the  absence  of  the  civet-cat  in  New 
York,  and  that  of  the  bobolink  or  the  Chinese  flying-fox  in 
California.     In  each  of  these  cases  there  is  no  evident  rea- 
son why  the  species  in  question  should  not  maintain  itself 
if  once  introduced.     The  fact  that  it  does  not  exist  is,  in 
general,  an  evidence  that  it  has  never  passed  the  barriers 
which  separate  the  region  in  question  from  its  original 
home. 

Local  illustrations  of  the  same  kind  may  be  found  in 
most  mountainous  regions.  In  the  Yosemite  Valley  in 
California,  for  example,  the  trout  ascend  the  Merced  River 
to  the  base  of  a  vertical  fall.  They  can  not  rise  above  this, 
and  so  the  streams  and  lakes  above  this  fall  are  destitute 
of  fish. 


GEOGRAPHICAL  DISTRIBUTION  OF  ANIMALS      275 

151.  Species  debarred  by  inability  to  maintain  their  ground. 

— Examples  of  the  second  class  are  seen  in  animals  which 
man  has  introduced  from  one  country  to  another.  The 
nightingale,  the  starling,  and  the  skylark  of  Europe  have 
been  repeatedly  set  free  in  the  United  States.  But  none  of 
these  colonies  has  long  endured,  perhaps  from  lack  of  adap- 
tation to  the  climate,  more  likely  from  severity  of  competi- 
tion with  other  birds.  In  other  cases  the  introduced  species 
has  been  better  fitted  for  the  conditions  of  life  than  the 
native  forms  themselves,  and  so  has  graduallv  crowded  out 
the  latter.  Both  these  cases  are  illustrated  among  the  rats. 
The  black  rat,  first  introduced  into  America  from  Europe 
about  1544,  helped  crowd  out  the  native  rats,  while  the 
brown  rat,  brought  in  still  later,  about  1775,  in  turn  practi- 
cally exterminated  the  black  rat,  its  fitness  for  the  condi- 
tions of  life  here  being  still  greater  than  that  of  the  other 
European  species. 

Certain  animals  have  followed  man  from  land  to  land, 
having  been  introduced  by  him  against  his  will  and  to  the 
detriment  of  his  domestic  animals  or  crops.  To  many  of 
these  the  term  vermin  has  been  applied.  Among  the  ver- 
min or  "  animal  weeds "  are  certain  of  the  rodents  (rats, 
mice,  rabbits,  etc.),  the  mongoose  of  India,  the  English 
sparrow,  and  many  kinds  of  noxious  insects.  Of  all  the 
vermin  of  this  class  few  have  caused  such  widespread  de- 
struction of  property  as  the  common  European  rabbit  intro- 
duced into  Australia.  The  annual  loss  through  its  presence 
is  estimated  at  $3,500,000. 

It  often  happens  that  man  himself  so  changes  the  en- 
vironment of  a  species  that  it  can  no  longer  maintain  it- 
self. Checking  the  increase  of  a  species,  either  by  actually 
killing  off  its  members  or  by  adverse  change  in  its  sur- 
roundings, is  to  begin  the  process  of  its  destruction.  Cir- 
cumstances become  unfavorable  to  the  growth  or  reproduc- 
tion of  an  animal.  Its  numbers  are  reduced,  fewer  are 
born  each  year,  and  fewer  reach  maturity,  it  grows  rare, 


276  ANIMAL  LIFE 

is  gone,  and  the  final  step  of  extinction  may  often  pass 
unnoticed. 

But  a  few  years  ago  the  air  in  the  Ohio  Valley  was  dark 
in  the  season  of  migration  with  the  hordes  of  passenger 
pigeons.  The  advance  of  a  tree-destroying,  pigeon-shooting 
civilization  has  gone  steadily  on,  and  now  the  bird  which 
once  crowded  our  Western  forests  is  in  the  same  region  an 
ornithological  curiosity.  The  extinction  of  the  American 
hison  or  "  buffalo,"  and  the  growing  rarity  of  the  grizzly 
bear,  the  wolf,  and  of  large  carnivora  generally,  furnishes 
cases  in  point.  When  Bering  and  Steller  landed  on  the 
Commander  Islands  in  1741,  the  sea-cow,  a  large  herbivo- 
rous creature  of  the  shores,  was  abundant  there.  In  about 
fifty  years  the  species,  being  used  for  food  by  fishermen, 
entirely  disappeared.  In  most  cases,  however,  a  species 
that  crosses  its  limiting  barriers,  but  is  unable  to  main- 
tain itself,  leaves  no  record  of  the  occurrence.  We  know,  as 
a  matter  of  fact,  that  stray  individuals  are  very  often  found 
outside  the  usual  limit  of  a  species.  A  tropical  bird  may 
be  found  in  New  Jersey,  a  tropical  fish  on  Cape  Cod,  or  a 
bird  from  Europe  on  the  shores  of  Maine.  Of  course, 
hundreds  of  other  cases  of  this  sort  must  escape  notice ; 
but,  for  one  reason  or  another,  the  great  majority  of  these 
waifs  are  unable  to  gain  a  new  foothold.  For  this  reason, 
outside  of  the  disturbances  created  by  man,  the  geographical 
distribution  of  species  changes  but  little  from  century  to 
century ;  and  yet,  when  we  study  the  facts  more  closely, 
evidences  of  change  appear  everywhere. 

152.  Species  altered  by  adaptation  to  new  conditions. 

Of  the  third  class  or  species  altered  in  a  new  environment 
examples  are  numerous,  but  in  most  cases  the  causes  in- 
volved can  only  be  inferred  from  their  effects.  One  class 
of  illustrations  may  be  taken  from  island  faunse.  An  island 
is  set  off  from  the  mainland  by  barriers  which  species  of 
land  animals  can  very  rarely  cross.  On  an  island  a  few  waifs 
of  wave  and  storm  may  maintain  themselves,  increasing  in 


JTie.  168— The  manatee,  or  sea-cow  ( Trichechus  latirostris).    A  living  species  of  sea- 
cow  related  to  the  now  extinct  Steller's  sea-cow. 


278 


ANIMAL  LIFE 


Fio.  169.— On  the  shore  of  Narborongh  Island,  one  of  the  Galapagos  Islands,  Pacific 
Ocean,  showing  peculiar  species  of  sea-lions,  lizards,  and  cormorants.  Drawn 
from  a  photograph  made  by  Messrs.  SNODGBASS  and  HELLER. 

numbers  so  as  to  occupy  the  territory;  but  in  so  doing 
only  those  will  survive  that  can  fit  themselves  to  the  new 
conditions.  Through  this  process  a  new  species  will  be 
formed,  like  the  parent  species  in  general  structure,  but 
having  gained  new  traits  adjusted  to  the  new  environment. 


GEOGRAPHICAL  DISTRIBUTION  OF  ANIMALS      279 

The  Galapagos  Islands  are  a  cluster  of  volcanic  rocks 
lying  in  the  open  sea  about  six  hundred  miles  to  the  west 
of  Ecuador.  On  these  islands  is  a  peculiar  land  fauna,  de- 
rived from  South  American  stock,  but  mostly  different  in 
species.  Darwin  noted  there  "twenty-six  land  birds;  of 
these,  twenty-one,  or  perhaps  twenty-three,  are  ranked  as 
distinct  species.  Yet  the  close  affinity  of  most  of  these 
birds  to  American  species  is  manifest  in  every  character,  in 
their  habits,  gestures,  and  tones  of  voice.7' 

Among  land  animals  similar  migrations  may  occur,  giv- 
ing rise,  through  the  adaptation  to  new  conditions,  to  new 
species.  The  separation  of  species  of  animals  isolated  in 
river  basins  or  lakes  often  permits  the  acquisition  of  new 
characters,  which  is  the  formation  of  distinct  species  in 
similar  fashion.  On  the  west  side  of  Mount  Whitney,  the 
highest  mountain  in  the  Sierra  Nevada  of  California,  there 
is  a  little  stream  called  Volcano  Creek.  In  this  brook  is  a 
distinct  species  or  form  of  trout,  locally  called  golden 
trout.  It  is  unusually  small,  very  brilliantly  colored,  its 
fins  being  bright  golden,  and  its  tiny  scales  scarcely  over- 
lap each  other  along  its  sides.  This  stream  flows  over  a 
high  waterfall  (Agua  Bonita)  into  the  Kern  Eiver.  The 
Kern  River  is  full  of  trout,  of  a  kind  (Salmo  gilberti)  to 
which  the  golden  trout  is  most  closely  allied.  There  can 
not  be  much  doubt  that  the  latter  is  descended  from  the 
former.  With  this  assumption,  it  is  easy  to  suppose  that 
once  the  waterfall  did  not  exist,  or  that  through  some 
agency  we  can  not  now  identify  certain  fishes  had  been 
carried  over  it.  Once  above  it,  they  can  not  now  return, 
nor  can  they  mix  with  the  common  stock  of  the  river. 
Those  best  adapted  to  the  little  stream  have  survived. 
The  process  of  adaptation  has  gone  on  till  at  last  a  distinct 
species  (or  sub-species  *)  is  formed.  In  recent  times  the 

*  In  descriptive  works  the  name  species  is  applied  to  a  form  when 
the  process  of  adaptation  seems  complete.  When  it  is  incomplete,  or 


FIG.  171. — Three  species  of  jack-rabbits,  differing  in  size,  color,  and  markings,  but 
believed  to  be  derived  from  a  common  stock.  The  differences  have  arisen 
through  isolation  and  adaptation.  The  upper  figure  shows  the  head  and  fore  legs 
of  the  black  jack-rabbit  (Lepus  insularis),  of  Espiritu  Santo  Island,  Gulf  of 
California ;  the  lower  right-hand  figure,  the  Arizona  jack-rabbit  (Lepus  alleni), 
specimen  from  Fort  Lowell,  Arizona  ;  and  the  lower  left-hand  figure  is  the  San 
Pedro  Martir  jack-rabbit  (Lepus  martirensis),  from  San  Pedro  Martir,  Baja 
California. 


282 


ANIMAL  LIFE 


hand  of  man  has  carried  the  golden  trout  to  other  little 
mountain  torrents,  where  it  thrives  as  well  as  in  the  one 
where  its  peculiarities  were  first  acquired. 

Other  cases  of  this  nature  are  found  among  the  blind 
fishes  of  the  caves  in  different  parts  of  the  world  (Fig.  172). 

In  general,  caves  are 
formed  by  the  ero- 
sion or  wearing  of 
underground  rivers. 
These  streams  are 
usually  clear  and  cold, 
and  when  they  issue 
to  the  surface  those 
fishes  that  like  cold 
and  shaded  waters 
are  likely  to  enter 
them.  But  to  have 
eyes  in  absolute  dark- 
ness, in  which  no  use 
can  be  made  of  them, 
is  a  disadvantage  in 
the  struggle  for  life. 
Hence  the  eyed  species  die  or  withdraw,  while  those  in  which 
the  eye  grows  less  from  generation  to  generation,  until  its 
function  is  finally  lost,  are  the  ones  which  survive.  By  such 
processes  the  blind  fishes  in  the  limestone  caves  of  Ken- 
tucky, Indiana,  Tennessee,  and  Missouri  have  been  formed. 


FIG.  172. — Fishes  showing  stages  in  the  loss  of  eyes 
and  color.  A,  Dismal  Swamp  fish  (Chologaster 
avitus),  ancestor  of  the  blind  fish  ;  B,  Agassiz's 
cave  fish  ( Chologaster  agassizi) ;  C,  cave  blind 
fish  (Typhlichthys  subterr emeus). 


rather  when  specimens  showing  intergradation  of  characters  are  known, 
the  word  sub-species  is  used.  The  word  variety  has  much  the  same 
meaning  when  used  for  a  subdivision  of  a  species,  but  it  is  a  term 
defined  with  less  exactness.  Thus  the  common  fox  ( Vulpes  pennsyl- 
vanicus)  is  a  distinct  species,  being  separate  from  the  arctic  fox  or  the 
gray  fox  or  the  fox  of  Europe.  The  cross  fox  ( Vulpes  pennsylvanicus 
decussatus)  is  called  a  sub-species,  as  is  the  silver  fox  ( Vulpes  pennsyl- 
vanicus argentatus),  because  these  intergrade  perfectly  with  the  common 
red  fox. 


GEOGRAPHICAL  DISTRIBUTION  OP  ANIMALS      283 

To  processes  of  this  kind,  on  a  larger  or  smaller  scale, 
the  variety  in  the  animal  life  of  the  globe  is  very  largely 
due.  Isolation  and  adaptation  give  the  clew  to  the  forma- 
tion of  a  very  large  proportion  of  the  "  new  species "  in 
any  group. 

153.  Effect  of  barriers. — It  will  be  thus  seen  that  geo- 
graphical distribution  is  primarily  dependent  on  barriers  or 
checks  to  the  movement  of  animals.       The  obstacles  met 
in  the  spread  of  animals  determine  the  limits  of  the  spe- 
cies.    Each  species  broadens  its  range  as  far  as  it  can.     It 
attempts  unwittingly,  through  natural  processes  of  increase, 
to  overcome  the  obstacles  of  ocean  or  river,  of  mountain  or 
plain,  of  woodland  or  prairie  or  desert,  of  cold  or  heat,  of 
lack  of  food  or  abundance  of  enemies — whatever  the  bar- 
riers may  be.     Were  it  not  for  these  barriers,  each  type  or 
species  would  become  cosmopolitan  or  universal.     Man  is 
pre-eminently  a  barrier-crossing  animal.    Hence  he  is  found 
in  all  regions  where  human  life  is  possible.     The  different 
races  of  men,  however,  find  checks  and  barriers  entirely 
similar  in  nature  to  those  experienced  by  the  lower  animals, 
and  the  race  peculiarities  are  wholly  similar  to  characters 
acquired  by  new  species  under  adaptation  to  changed  con- 
ditions.    The  degree  of  hindrance  offered  by  any  barrier 
differs  with  the  nature  of  the  species  trying  to  surmount  it. 
That  which  constitutes  an  impassable  obstacle  to  one  form 
may  be  a  great  aid  to  another.     The  river  which  blocks  the 
monkey  or  the  cat  is  the  highway  of  the  fish  or  the  turtle. 
The  waterfall  which  limits  the  ascent  of  the  fish  is  the 
chosen  home  of  the  ouzel.     The  mountain  barrier  which 
the  bobolink  or  the  prairie-dog  does  not  cross  may  be  the 
center  of  distribution  of  the  chief  hare  or  the  arctic  blue- 
bird. 

154.  Relation  of  species  to  habitat.— The  habitat  of  a 
species  of  animal  is  the  jegion  in  which  it  is  found  in  a 
state  of  Nature.     It  is  currently  believed  that  the  habitat 
of  any  creature  is  the  region  for  which  it  is  best  adapted. 


284  ANIMAL  LIFE 

But  the  reverse  of  this  is  often  true.  There  are  many  cases 
in  which  a  species  introduced  in  a  new  territory,  through 
the  voluntary  or  involuntary  influence  of  man,  has  shown  a 
marvelous  adaptation  and  power  of  persistence.  The  rapid 
spread  of  rabbits  and  pigs  as  wild  animals  in  Australia,  of 
horses  and  cattle  in  South  America,  and  of  the  English 
sparrow  in  North  America,  of  bumble-bees  and  house- 
flies  in  New  Zealand,  are  illustrations  of  this.  Not  one 
of  these  animals  has  maintained  itself  in  the  wild  state 
in  its  native  land  as  successfully  as  in  these  new  countries 
to  which  it  has  been  introduced.  The  work  of  introduc- 
tion of  useful  animals  illustrates  the  same  fact.  The  shad, 
striped  bass,  and  cat-fish  from  the  Potomac  River,  intro- 
duced into  the  Sacramento  River  and  its  tributaries  by  the 
United  States  Fish  Commission,  are  examples  in  point. 
These  valued  food-fishes  are  nowhere  more  at  home  than  in 
the  new  waters  where  no  species  of  their  types  had  ever 
existed  before.  The  carp,  originally  brought  to  Europe 
from  China,  and  thence  to  the  United  States  as  a  food- 
fish,  becomes  in  California  a  nuisance,  which-  can  not  be 
eradicated,  destroying  the  eggs  and  the  foodstuff  of  far 
better  fish. 

In  all  mountain  regions  waterfalls  are  likely  to  occur, 
and  these  serve  as  barriers,  preventing  the  ascent  of  trout 
and  other  fishes.  On  this  account  in  the  mountains  of  Cali- 
fornia, Colorado,  Wyoming,  and  other  States,  hundreds  of 
lakes  and  streams  suitable  for  trout  are  found  in  which  no 
fishes  ever  exist.  In  the  Yellowstone  Park  this  fact  is  es- 
pecially noticeable.  This  region  is  a  high  volcanic  plateau, 
formed  by  the  filling  of  an  ancient  granite  basin  with  a  vast 
deposit  of  lava.  The  streams  of  the  park  are  very  cold  and 
clear,  in  every  way  favorable  for  the  growth  of  trout ;  yet, 
with  the  exception  of  a  single  stream,  the  Yellowstone 
River,  none  of  the  streams  was  found  to  contain  any  fish 
in  that  part  of  it  lying  on  the  plateau.  Below  the  plateau 
all  of  them  are  w^ll  stocked.  The  reason  for  this  is  ap- 


Is 


1! 
§§. 

ll 


286  -  ANIMAL  LIFE 

parent  in  the  fact  that  the  plateau  is  fringed  with  cataracts 
which  fishes  can  not  ascend.  Each  stream  has  a  canon  or 
deep  gorge  with  a  waterfall  at  its  head,  near  the  point 
where  it  leaves  the  hard  bed  of  black  lava  for  the  rock 
below  (Fig.  173).  So  for  an  area  of  fifteen  hundred  square 
miles  within  the  Yellowstone  National  Park  the  streams 
were  without  trout  because  their  natural  inhabitants  had 
never  been  able  to  reach  them.  When  this  state  of  things 
was  discovered  it  was  easy  to  apply  the  remedy.  Trout  of 
different  species  were  carried  above  the  cascades,  and  these 
have  multiplied  with  great  rapidity. 

The  exception  noted  above,  that  of  the  Yellowstone 
River  itself,  evidently  needs  explanation.  An  abundance 
of  trout  is  found  in  this  river  both  above  and  below  the 
great  falls,  and  no  other  fish  occurs  with  it.  This  anomaly 
of  distribution  is  readily  explained  by  a  study  of  the  tribu- 
taries at  the  head  waters  of  the  river.  When  we  ascend 
above  Yellowstone  Lake  to  the  continental  divide,  we  find 
on  its  very  summit  that  only  about  an  eighth  of  a  mile  of 
wet  meadow  and  marsh,  known  as  Two  Ocean  Pass  (Fig. 
174),  separates  the  drainage  of  the  Yellowstone  from  that 
of  the  Columbia.  A  stream  known  as  Atlantic  Creek  flows 
into  the  Yellowstone,  while  the  waters  of  Pacific  Creek  on 
the  other  side  find  their  way  into  the  Snake  River.  These 
two  creeks  are  connected  by  waterways  in  the  wet  meadow, 
and  trout  may  pass  from  one  to  the  other  without  check. 
Thus  from  the  Snake  River  the  Yellowstone  received  its 
trout,  and  from  the  Yellowstone  they  have  spread  to  the 
streams  tributary  to  the  upper  Missouri. 

This  case  is  a  type  of  the  anomalies  in  distribution  of 
which  the  student  of  zoogeography  will  find  many.  But 
each  effect  depends  upon  some  cause,  and  a  thorough  study 
of  the  surroundings  or  history  of  a  species  will  show  what 
this  cause  may  be.  In  numerous  cases  in  which  fishes  have 
been  found  above  an  insurmountable  cascade,  the  cause  is 
seen  in  a  marsh  flooded  at  high  water,  connecting  one 


II 


2  * 


11 

ge  « 


288  ANIMAL  LIFE 

drainage  basin  with  another.  An  example  of  this  is  found 
in  Lava  Creek  in  Yellowstone  Park.  Above  Undine  and 
Wraith  Falls,  both  insurmountable,  are  found  an  abun- 
dance of  trout.  A  marsh  dry  in  summer  connects  Lava 
Creek  with  Black  Tail  Deer  Creek,  a  tributary  of  the 
Yellowstone  and  without  waterfall.  From  the  Yellow- 
stone through  this  creek  and  marsh  the  trout  find  their 
way  into  Lava  Creek.  In  California  numerous  anomalies 
have  been  noted,  as  the  occurrence  of  Tahoe  trout  in 
Feather  Eiver  and  in  the  Blue  Lakes  of  Amador,  which  are 
on  the  other  side  of  the  main  crest  of  the  Sierra  Nevada 
from  Lake  Tahoe,  and  the  occurrence  of  the  Whitney 
golden  trout  in  Lone  Pine  Creek,  another  similar  instance. 
In  each  case  naturalists  have  found  the  man  who  actually 
carried  the  species  across  the  divide.  If  this  matter  had 
been  investigated  a  generation  later,  these  cases  would  have 
been  unexplainable  anomalies  in  geographical  distribution. 
Eeal  causes  are  almost  always  simple  when  they  are  once 
known. 

The  ways  in  which  species  may  cross  barriers  in  a  state 
of  Nature  are  as  varied  as  the  creatures  themselves,  and  far 
more  varied  than  the  actual  barriers.  By  the  long-con- 
tinued process  of  adjustment  to  conditions  with  the  inces- 
sant destruction  of  the  unadapted,  the  various  organisms 
have  become  so  well  fitted  to  their  surroundings  that  the 
casual  observer  may  well  suppose  that  each  inhabits  the 
region  best  fitted  for  it.  Men  have  even  thought  that  the 
conditions  of  life  have  been  fitted  to  the  creatures  them- 
selves, so  perfect  is  this  relation. 

155.  Character  of  barriers  to  distribution. — Taking  the 
animal  kingdom  as  a  whole,  the  two  great  barriers  modify- 
ing distribution  are  the  presence  of  the  sea  and  changes  in 
temperature.  It  is  only  in  rare  cases  that  any  land  ani- 
mals can  cross  either  of  the  great  oceans,  and  these  rare 
cases  relate  chiefly  to  the  arctic  regions.  For  this  reason 
the  land  faunae  of  Africa,  South  America,  and  Australia 


GEOGRAPHICAL  DISTRIBUTION  OF  ANIMALS      289 

have  developed  almost  independently  of  one  another.     To 
the  fresh-water  fishes  the  sea  forms  equally  a  barrier,  and 


even  the  shore-fishes  very  rarely  pass  across  great  depths. 
Relatively  few  of  the  shore-fishes  of  Cuba,  for  example,  ever 
cross  the  deep  Florida  Straits,  and  none  of  those  of  Cali- 
20 


290  ANIMAL  LIFE 

fornia  ever  reach  Honolulu,  nor  are  Hawaiian  shore-fishes 
ever  seen  on  the  coast  of  California.  For  these  reasons 
natural  boundaries  of  the  great  realms  of  distribution  are 
found  in  the  sea. 

The  other  great  check  to  distribution  is  found  in  heat 
and  cold.  Most  of  the  tropical  animals  can  not  endure 
frost.  The  arctic  animals,  however  fierce  or  active,  are 
enfeebled  by  heat.  The  timber  line,  north  of  which  and 
above  which  frost  occurs  the  year  round,  therefore  serves 


PIG.  176. — Alligators ;  animals  found  only  in  the  warm  waters  of  tropical  and  sub- 
tropical regions. 

as  a  boundary  of  limitation.  Another  equally  marked  is 
the  frost  line.  Even  the  fishes  of  the  tropics  are  extreme- 
ly sensitive  to  slight  cold.  Off  Florida  Keys  the  cutlass- 
fish  is  sometimes  seen  stiff  and  benumbed  on  the  water, 
where  the  temperature  is  scarcely  below  60°  Fahr.  A 
"norther"  on  the  Gulf  of  Mexico  will  sometimes  bring 
fishes  which  live  in  considerable  depths  to  the  surface, 
through  chilling  the  water.  These  barriers  are  rarely 
crossed  by  localized  species,  but  many  forms,  especially 
birds,  keep  within  a  relatively  uniform  temperature  through 
migration.  The  summers  are  spent  in  the  north  or  in  the 
mountains,  the  winters  in  districts  that  are  warmer. 


GEOGRAPHICAL  DISTRIBUTION  OF  ANIMALS      291 

The  climate,  as  distinct  from  the  temperature,  also 
greatly  influences  many  species.  In  the  Eastern  United 
States  and  in  the  extreme  Northwest,  as  in  Europe  and 
much  of  Asia,  the  atmosphere  is  humid  all  the  year  long. 
Eains  occur  at  intervals  in  the  summer,  and  rain  or  snow  in 
the  winter.  The  green  season  is  from  spring  to  fall,  and  the 
resting  of  plants  is  in  the  winter.  To  this  condition  the 
native  animals  adapt  themselves,  and  this  would  seem  to 
be  the  natural  order  of  things. 

But  as  we  pass  the  Western  plains  of  Nebraska,  Kan- 
sas, and  Texas  this  condition  is  materially  changed.  For 
part  of  the  year  rainfall  is  practically  unknown.  The  air 
becomes  dry,  and  under  the  cloudless  sky  the  greater  part 
of  the  vegetation  ripens  its  seed  and  perishes.  This  is  the 
arid  climate.  When  the  rainfall  is  very  scant  the  region 
is  never  covered  with  verdure,  and  is  known  as  desert. 
Such  great  desert  tracts  are  found  in  parts  of  Wyoming, 
Utah,  Nevada,  Idaho,  Colorado,  Arizona,  New  Mexico,  Cali- 
fornia, as  well  as  in  the  northern  parts  of  Mexico.  In  some 
cases  the  deserts  are  exposed  to  great  heat,  forming  an 
ultra-torrid  region,  as  in  Death  Valley  in  California  and  in 
certain  parts  of  Arizona. 

But  the  arid  region  is  not  as  a  whole  desolate.  In  many 
parts  rain  falls  more  or  less  heavily  for  part  of  the  year, 
bringing  a  rank  growth  of  annual  grasses  and  of  verdure 
in  general.  In  California  this  rainfall  is  in  the  winter,  the 
coldest  part  of  the  year,  and  the  country  is  green  from 
November  or  October  to  June  or  May.  In  Mexico  and 
northward  to  Colorado  the  chief  rainfall  is  in  midsummer, 
the  warmest  part  of  the  year,  and  the  summer  is  the  time 
of  verdure. 

To  all  these  conditions  the  plant  life  must  adapt  itself 
and  with  this  the  animal  life.  But  the  species  that  have 
become  fitted  to  the  arid  habitat  have  undergone  some 
change  in  the  process  and  may  have  become  different  spe- 
cies. It  is,  then,  not  easy  for  them  to  recross  the  barrier 


292  ANIMAL  LIFE 

of  climate  to  compete  with  those  forms  already  adapted. 
For  this  reason  a  marked  change  of  climate  like  a  marked 
change  of  temperature  forms  a  natural  barrier  to  distribu- 
tion and  serves  to  circumscribe  a  natural  fauna. 

Closely  associated  with  climate  is  the  nature  of  forest 
growth,  the  growth  of  grass,  and  in  general  the  development 
of  conditions  which  serve  for  food  or  shelter  to  animals. 
These  conditions  depend  in  part  on  soil,  partly  on  climate 
and  topography,  and  partly  on  the  acts  of  man.  The  for- 
est and  forest  soils,  acting  like  a  great  sponge,  retain  the 
waters  of  the  rainy  season,  and  thus  regulate  the  size  of 
the  streams.  The  stream  that  changes  least  in  volume  is 
most  favorable  to  the  life  of  fishes,  frogs,  and  water  ani- 
mals generally.  The  destruction  of  forests  on  the  moun- 
tain sides  acts  adversely  to  the  life  of  these  creatures  as 
well  as  to  the  interests  of  the  farmer  below  whose  lands 
the  streams  should  water.  When  the  forests  are  destroyed, 
the  great  host  of  wood  creatures,  the  bears,  squirrels,  war- 
blers, various  birds,  beasts,  and  insects  of  the  woods  can  no 
longer  maintain  themselves,  and  grow  rare  and  disappear. 
For  reasons  that  are  obvious  the  conditions  that  produce 
forest,  prairie,  canebrake,  sage -desert,  cactus  -  desert,  and 
the  like  are  potent  in  regulating  the  distribution  of  the 
species. 

Still  another  set  of  conditions  depends  on  the  food  sup- 
ply. The  planting  of  orchards  tends  to  multiply  greatly 
the  number  of  individuals  of  those  species  which  prey  upon 
fruit.  When  food  is  abundant  the  severity  of  the  struggle 
for  life  is  relaxed  and  individuals  increase  in  number.  A 
species  may  be  put  to  great  stress  by  the  disappearance  of 
the  animal  or  plant  on  which  it  has  depended.  Each 
change  made  by  man  among  the  wild  animals  or  plants 
may  have  far-reaching  effects  upon  others.  The  coyote  or 
prairie-wolf  destroys  sheep  in  the  ranges  of  the  West.  It 
is  thinned  out  by  means  of  the  bounty  upon  its  scalp. 
Then  the  jack-rabbit,  on  which  it  also  feeds,  greatly  in- 


GEOGRAPHICAL  DISTRIBUTION  OF  ANIMALS      293 

creases  in  abundance,  injuring  fruit  trees  and  grain  fields. 
It  is  then  necessary  to  pay  for  its  destruction  also. 

To  destroy  hawks  or  owls  because  they  catch  chickens 
may  increase  the  numbers  and  destructiveness  of  field-mice 
on  which  they  also  prey.  To  shoot  robins,  linnets,  and 
other  birds  that  destroy  small  fruits  is  likely  to  increase 
greatly  the  insect  pests  on  which  these  birds  also  feed. 
The  inter-relations  of  species  and  species  are  so  close  that 
none  should  be  exterminated  by  man  unless  its  habits  and 
relations  have  been  subjected  to  careful  scientific  study. 
Still  less  should  any  new  ones  be  introduced  without  the 
fullest  consideration  of  the  possible  results.  For  example, 
the  mongoose,  a  weasel-like  creature,  was  introduced  from 
India  into  Jamaica  to  kill  rats  and  mice.  It  killed  also  the 
lizards,  and  thus  produced  a  plague  of  fleas,  an  insect  which 
the  lizards  kept  in  check.  The  English  sparrow,  intro- 
duced that  it  might  feed  on  insects  inhabiting  shade-trees, 
has  become  a  nuisance,  crowding  out  better  birds  and  not 
accomplishing  the  purpose  for  which  it  was  brought  to  the 
United  States. 

To  most  kinds  of  animals  a  mountain  range  must  act  as 
a  barrier  to  distribution.  In  a  region  having  high  moun- 
tains a  species  will  become  in  time  split  up  into  several, 
because  the  individuals  in  one  valley  will  be  isolated  from 
those  of  another.  The  fauna  of  California  furnishes  many 
illustrations  of  this,  as  among  its  mountain  chains  are 
many  deep  valleys  shut  off  from  each  other  and  having 
different  peculiarities  of  temperature.  For  this  reason  two 
counties  of  California  differ  much  more  widely  in  their 
fauna  than  do  two  counties  in  Illinois.  But  Illinois  as  a 
whole  has  more  different  kinds  of  animals  than  California, 
because  no  barrier  anywhere  prevents  their  entrance.  The 
State  has,  we  may  say,  its  doors  wide  open  to  immigrants 
from  all  quarters.  The  same  is  true  of  Iowa  or  of  Kansas 
or  Kentucky.  Illinois  has  a  richer  fauna  than  Iowa,  be- 
cause its  extension  is  north  and  south,  and  it  therefore 


294  ANIMAL  LIFE 

covers  a  wider  range  of  climate.  Kentucky  lias  a  richer 
fauna  than  Iowa  because  it  includes  a  greater  variety  of 
conditions.  New  England  was  called  by  Professor  Agassiz 
a  "  zoological  island,"  because  of  the  relatively  small  num- 
ber of  its  native  animals,  especially  of  species  inhabiting  its 
rivers.  The  cause  of  this  is  found  in  its  isolation,  being 
shut  off  from  the  Middle  States  by  mountain  ranges,  while 
it  is  bounded  on  two  sides  by  the  sea. 

156.  Barriers  affecting  fresh-water  animals. — The  animals 
inhabiting  fresh-water  streams  are  affected  by  differences  in 
temperature  and  elevation  much  as  land  animals  are.  They 
tend  to  spread  from  stream  to  stream  whenever  they  can 
find  their  way.  An  isolated  stream  is  likely  to  have  its 
peculiar  fauna  just  as  island  life  is  likely  to  differ  from 
that  of  the  mainland.  The  same  species  wanders  widely 
within  the  limits  of  a  single  river  basin.  If  a  kind  of  fish 
establishes  itself  anywhere  in  the  Mississippi  Valley,  it  may 
find  its  way  to  every  stream  in  the  whole  basin.  If  it  likes 
cold  spring  water,  as  the  rainbow-darter  does,  we  may  look 
for  it  in  any  cold  spring.  If,  like  the  long-eared  sun-fish, 
it  frequents  deep  pools  in  the  brooks,  we  may  look  for  it 
under  roots  of  stumps  and  in  every  "  swimming  hole."  If, 
like  the  channel-cat,  it  chooses  the  ripples  of  a  river,  we 
may  fish  for  it  wherever  ripples  are.  The  larger  the  whole 
river  basin  the  more  species  find  their  way  into  it,  and 
therefore  the  greater  the  number  of  species  in  any  one  of 
its  streams. 

Each  species  finds  its  habitat  fitted  to  its  life,  and  then 
in  turn  is  forced  to  adapt  itself  to  this  habitat.  Any  other 
kind  of  habitat  then  appears  as  a  barrier  to  its  distribu- 
tion. Thus  to  a  fish  of  the  ripples  a  stretch  of  still  water 
becomes  a  barrier.  A  species  adapted  to  sandy  bottoms 
will  seldom  force  its  way  through  swift  waters  or  among 
weeds  or  rocks.  The  effect  of  waterfalls  as  barriers  is  else- 
where noticed.  In  some  streams  the  dam  made  by  a  colony 
of  beavers  has  the  same  effect.  Mill-dams  and  artificial 


GEOGRAPHICAL  DISTRIBUTION  OF  ANIMALS      295 

waterfalls  have  checked  the  movements  of  many  species, 
while  others  have  been  helped  by  artificial  channels  or 
canals.  Streams  that  run  muddy  at  times  are  not  favor- 
able for  animal  life.  Still  less  favorable  is  the  condition 
frequent  in  the  arid  region  in  which  streams  are  full  to 
the  banks  in  the  rainy  season  and  shrunk  to  detached 
pools  in  the  dry  months. 

The  stream  that  has  the  greatest  variety  of  animals  in  it 
would  be  one  (1)  connected  with  a  large  river,  (2)  in  a  warm 
climate,  (3)  with  clear  water  and  (4)  little  fluctuation  from 
winter  to  summer,  (5)  with  little  change  in  the  clearness  of 
the  water,  (6)  a  gravelly  bottom,  (7)  preferably  of  lime- 
stone, and  (8)  covered  in  its  quiet  reaches  and  its  ripples 
with  water-weeds.  These  conditions  are  best  realized  in 
tributaries  of  the  Ohio,  Cumberland,  Tennessee,  and  Ozark 
Rivers  among  American  streams,  and  it  is  in  them  that  the 
greatest  number  of  species  of  fresh-water  animals  (fishes, 
cray-fishes,  mussels,  etc.)  has  been  recorded.  These  streams 
approach  most  nearly  to  the  ideal  homes  for  animals  of  the 
fresh  waters.  The  streams  of  Wisconsin,  Michigan,  and  the 
Columbia  region  have  many  advantages,  but  are  too  cold. 
Those  of  Illinois,  Iowa,  northern  Missouri,  and  Kansas  are 
too  sluggish,  and  sometimes  run  muddy.  Those  of  Texas 
and  California  shrink  too  much  in  summer,  and  are  too 
isolated  The  streams  of  the  Atlantic  coast  are  less  iso- 
lated, but  none  connect  with  a  great  basin,  and  those  of 
New  England  run  too  cold  for  the  great  mass  of  the  spe- 
cies. For  similar  reasons  the  fresh-water  animal  life  of 
Europe  is  relatively  scanty,  that  of  the  Danube  and  Volga 
being  richest.  The  animal  life  of  the  fresh  water  of  South 
America  centers  in  the  Amazon,  and  that  of  Africa  in  the 
Nile,  the  Niger,  and  the  Congo.  The  great  rivers  of  Si- 
beria, like  the  Yukon  in  Alaska  and  the  Mackenzie  River 
in  British  America,  have  but  few  forms  of  fresh- water  ani- 
mals, though  those  kinds  fitted  for  life  in  cold,  clear  water 
exist  in  great  abundance. 


296  ANIMAL   LIFE 

157.  Modes  of  distribution. — The  means  and  modes  of  mi- 
gration and  distribution  are  obvious  in  the  case  of  animals 
that  can  fly  or  swim  or  make  long  journeys  on  foot.     An 
island  can  be  visited  and  become  peopled  by  birds  from  the 
nearest  mainland.     Fishes  and  marine  mammals  can  travel 
from  ocean  to  ocean.     But  many  animals  have  no  means 
of  crossing  watery  barriers.     "  Oceanic  islands,  that  have 
been  formed  de  novo  in  mid-ocean  and  are  not  detached 
portions  of  pre-existing  continents,  are   almost  invariably 
free  from  such  animals  as  are  incapable  of  traversing  the 
sea.     If   sufficiently  distant  from  any  continent,  oceanic 
islands  are  generally  without  mammals,  reptiles,  and  am- 
phibia, but  have  both  birds  and  insects  and  certain  other 
invertebrates  which  are  transported  to  them  by  involuntary 
migration." 

As  suggested  in  the  last  sentence,  migration  may  be 
passive  or  involuntary.  For  example,  those  minute  ani- 
mals that  can  become  dried  up  and  yet  retain  the  power 
of  renewing  their  active  life  under  favorable  conditions  are 
sometimes  carried  in  the  dried  mud  adhering  to  the  feet  of 
birds,  and  may  thus  become  widely  distributed.  Parasites 
are  carried  by  their  hosts  in  all  their  wanderings.  Some 
animals,  as  rats  and  mice,  are  carried  by  ships  and  railway 
trains  and  thus  widely  distributed. 

158.  Fauna  and  fauna!  areas. — The  term  fauna  is  applied 
to  the  animals  of  any  region  considered  collectively.     Thus 
the  fauna  of  Illinois  comprises  the  entire  list  of  animals 
found  naturally  in  that  State.     It  includes  the  aboriginal 
men,  the  black  bear,  the  fox,  and  all  its  animal  life  down 
to  the  Ammba,.     The  relation  of  the  fauna  of  one  region 
to  that  of  another  depends  on  the  ease  with  which  bar- 
riers may  be  crossed.     Thus  the  fauna  of  Illinois  differs 
little  from  that  of  Indiana  or  Iowa,  because  the  State  con- 
tains no  barriers  that  animals  may  not  readily  pass.     On 
the  other  hand,  the  fauna  of  California  or  Colorado  differs 
materially  from  that  of  adjoining  regions,  because  a  moun- 


GEOGRAPHICAL  DISTRIBUTION  OF  ANIMALS      297 

tainous  country  is  full  of  barriers  which  obstruct  the  diffu- 
sion of  life.  Distinctness  is  in  direct  proportion  to  isola- 
tion. What  is  true  in  this  regard  of  the  fauna  of  any  region 
is  likewise  true  of  its  individual  species.  The  degree  of 
resemblance  among  individuals  is  in  strict  proportion  to  the 
freedom  of  their  movements.  Variation  within  the  limits 
of  a  species  is  again  proportionate  to  the  barriers  which 
prevent  equal  and  free  diffusion. 

159.  Realms  of  animal  life. — The  various  divisions  or 
realms  into  which  the  land  surface  of  the  earth  may  be 
divided  on  the  basis  of  the  character  of  animal  life  have 
their  boundary  in  the  obstacles  offered  to  the  spread  of  the 
average  animal.  In  spite  of  great  inequalities  in  this  regard, 
we  may  yet  roughly  divide  the  land  of  the  globe  into  seven 
principal  realms  or  areas  of  distribution,  each  limited  by 
barriers,  of  which  the  chief  are  the  presence  of  the  sea  and 
the  occurrence  of  frost.  There  are  the  Arctic,  North  Tem- 
perate, South  American,  Indo-African,  Lemurian,  Patago- 
nian,  and  Australian  realms.  Of  these  the  Australian 
realm  alone  is  sharply  defined.  Most  of  the  others  are  sur- 
rounded by  a  broad  fringe  of  debatable  ground  that  forms 
a  transition  to  some  other  zone. 

The  Arctic  realm  includes  all  the  land  area  north  of  the 
isotherm  of  32°.  Its  southern  boundary  corresponds  closely 
with  the  northern  limit  of  trees.  The  fauna  of  this  region 
is  very  homogeneous.  It  is  not  rich  in  species,  most  of  the 
common  types  of  life  of  warmer  regions  being  excluded. 
Among  the  large  animals  are  the  polar  bear,  the  walrus,  and 
certain  species  of  "  ice-riding  "  seals.  There  are  a  few  spe- 
cies of  fishes,  mostly  trout  and  sculpins,  and  a  few  insects. 
Some  of  these,  as  the  mosquito,  are  excessively  numerous 
in  individuals.  Reptiles  are  absent  from  this  region  and 
many  of  its  birds  migrate  southward  in  the  winter,  finding 
in  the  arctic  only  their  breeding  homes.  When  we  consider 
the  distribution  of  insects  and  other  small  animals  of  wide 
diffusion  we  must  add  to  the  arctic  realm  all  high  moun- 


GEOGRAPHICAL  DISTRIBUTION  OP  ANIMALS      299 

tains  of  other  realms  whose  summits  rise  above  the  timber 
line.  The  characteristic  large  animals  of  the  arctic,  as  the 
polar  bear  or  the  musk-ox  or  the  reindeer,  are  not  found 
there,  because  barriers  shut  them  off.  But  the  flora  of  the 
mountain  top,  even  under  the  equator,  may  be  character- 
istically arctic,  and  with  the  flowers  of  the  north  may  be 
found  the  northern  insects  on  whose  presence  the  flower 
depends  for  its  fertilization.  So  far  as  climate  is  concerned 
high  altitude  is  equivalent  to  high  latitude.  On  certain 
mountains  the  different  zones  of  altitude  and  the  corre- 
sponding zones  of  plant  and  insect  life  are  very  sharply 
defined  (Fig.  178). 

The  North  Temperate  realm  comprises  all  the  land  be- 
tween the  northern  limit  of  trees  and  the  southern  limit  of 
frost.  It  includes,  therefore,  nearly  the  whole  of  Europe, 
most  of  Asia,  and  most  of  North  America.  While  there 
are  large  differences  between  the  fauna  of  North  America 
and  that  of  Europe  and  Asia,  these  differences  are  of  minor 
importance  and  are  scarcely  greater  in  any  case  than  the 
difference  between  the  fauna  of  California  and  that  of  our 
Atlantic  coast.  The  close  union  of  Alaska  with  Siberia 
gives  the  arctic  region  an  almost  continuous  land  area  from 
Greenland  to  the  westward  around  to  Norway.  To  the 
south  everywhere  in  the  temperate  zone  realm  the  species 
increase  in  number  and  variety,  and  the  differences  between 
the  fauna  of  North  America  and  that  of  Europe  are  due  in 
part  to  the  northward  extension  into  the  one  and  the  other 
of  types  originating  in  the  tropics.  Especially  is  this  true 
of  certain  of  the  dominant  types  of  singing  birds.  The 
group  of  wood-warblers,  tanagers,  American  orioles,  vireos, 
mocking-birds,  with  the  fly-catchers  and  humming-birds  so 
characteristic  of  our  forests,  are  unrepresented  in  Europe. 
All  of  them  are  apparently  immigrants  from  the  neotropical 
realm  where  nearly  all  of  them  spend  the  winter.  In  the 
same  way  central  Asia  has  many  immigrants  from  the  Indian 
realm  to  the  southward.  With  all  these  variations  there 


GEOGRAPHICAL  DISTRIBUTION  OF  ANIMALS      3Q1 

is  an  essential  unity  of  life  over  this  vast  area,  and  the  rec- 
ognition of  North  America  as  a  separate  (nearctic)  realm, 
which  some  writers  have  attempted,  seems  hardly  practi- 
cable. 

The  Neotropical  or  South  American  realm  includes 
South  America,  the  West  Indies,  the  hot  coast  lands  of 
Mexico,  and  those  parts  of  Florida  and  Texas  where  frost 
does  not  occur.  Its  boundaries  through  Mexico  are  not 
sharply  denned,  and  there  is  much  overlapping  of  the  north 
temperate  realm  along  its  northern  limit.  Its  birds  espe- 
cially range  widely  through  the  United  States  in  the  sum- 
mer migrations,  and  a  large  part  of  them  find  in  the  North 
their  breeding  home.  Southward,  the  broad  barrier  of  the 
two  oceans  keeps  the  South  American  fauna  very  distinct 
from  that  of  Africa  or  Australia.  The  neotropical  fauna  is 
richest  of  all  in  species.  The  great  forests  of  the  Amazon 
are  the  dreams  of  the  naturalists.  Characteristic  types 
among  the  larger  animals  are  the  snout  or  broad-nosed 
(platyrrhine)  monkeys,  which  in  many  ways  are  very  distinct 
from  the  monkeys  and  apes  of  the  Old  World.  In  many  of 
them  the  tip  of  the  tail  is  highly  specialized  and  is  used  as 
a  hand.  The  Edentates  (armadillos,  ant-eaters,  etc.)  are 
characteristically  South  American,  and  there  are  many 
peculiar  types  of  birds,  reptiles,  fishes,  and  insects. 

The  Indo- African  realm  corresponds  to  the  neotropical 
realm  in  position.  It  includes  the  greater  part  of  Africa, 
merging  gradually  northward  into  the  north  temperate 
realm  through  the  transition  districts  which  border  the 
Mediterranean.  It  includes  also  Arabia,  India,  and  the 
neighboring  islands,  all  that  part  of  Asia  south  of  the  limit 
of  frost.  In  monkeys,  carnivora,  ungulates,  and  reptiles 
this  region  is  wonderfully  rich.  In  variety  of  birds,  fishes, 
and  insects  the  neotropical  realm  exceeds  it.  The  monkeys 
of  this  district  are  all  of  the  narrow-nosed  (catarrhine) 
type,  various  forms  being  much  more  nearly  related  to 
man  than  is  the  case  with  the  peculiar  monkeys  of  South 


302 


ANIMAL  LIFE 


America.  Some  of  these  (anthropoid  apes)  have  much 
in  common  with  man,  and  a  primitive  man  derived  from 
these  has  been  imagined  by  Haeckel  and  others.  No 
creature  of  this  character  is  yet  known,  but  that  it  may 
have  once  existed  is  not  impossible.  To  this  region  be- 
long the  elephant,  the  rhinoceros,  and  the  hippopotamus, 
as  well  as  the  lion,  tiger,  leopard,  giraffe,  the  wild  asses, 
and  horses  of  various  species,  besides  a  large  number  of 
ruminant  animals  not  found  in  other  parts  of  the  world. 
It  is,  in  fact,  in  its  lower  mammals  and  reptiles  that  its 

most  striking  dis- 
tinctive characters 
are  found.  In  its 
fish  fauna  it  has 
very  much  in  com- 
mon with  South 
America. 

The  Lemurian 
realm  comprises 
Madagascar  alone. 
It  is  an  isolated  di- 
vision of  the  Indo- 
African  realm,  but 
the  presence  of 
many  species  of 
lemur  and  an  un- 
specialized  or 
primitive  type  of 
lemur  is  held  to 
justify  its  recogni- 
tion as  a  distinct 

realm.  In  most  other  groups  of  animals  the  fauna  of  Mada- 
gascar is  essentially  that  of  neighboring  parts  of  Africa. 

The  Patagonian  realm  includes  the  south  temperate 
zone  of  South  America.  It  has  much  in  common  with  the 
neotropical  realm  from  which  its  fauna  is  mainly  derived, 


FIG.  179. — A  lemur  (Lemur  varius). 


GEOGRAPHICAL  DISTRIBUTION  OF  ANIMALS      303 

but  the  presence  of  frost  is  a  barrier  which  vast  numbers  of 
species  can  not  cross.  Beyond  the  Patagonian  realm  lies 
the  Antarctic  continent.  The  scanty  fauna  of  this  region 
is  little  known,  and  it  probably  differs  from  the  Patagonian 
fauna  chiefly  in  the  absence  of  all  but  the  ice-riding  species. 

The  Australian  realm  comprises  Australia  and  the 
neighboring  islands.  It  is  more  isolated  than  any  of  the 
others,  having  been  protected  by  the  sea  from  the  invasions 
of  the  characteristic  animals  of  the  Indo- African  and  tem- 
perate realms.  It  shows  a  singular  persistence  of  low  or 
primitive  types  of  vertebrate  life,  as  though  in  the  process 
of  evolution  the  region  had  been  left  a  whole  geological 
age  behind  the  others.  It  is  certain  that  if  the  closely 
competing  fauna  of  Africa  and  India  could  have  been  able 
to  invade  Australia,  the  dominant  mammals  and  birds  of 
that  region  would  not  have  been  left  as  they  are  now — mar- 
supials and  parrots. 

It  is  only  when  barriers  have  shut  out  competition  that 
simple  or  unspecialized  types  abound.  The  larger  the  land 
area  and  the  more  varied  its  surface,  the  greater  is  the 
stress  of  competition  and  the  more  specialized  are  its  char- 
acteristic forms.  As  part  of  this  specialization  is  in  the 
direction  of  hardiness  and  power  to  persist,  the  species  from 
the  large  areas,  as  a  whole,  are  least  easy  of  extermination. 
The  rapid  multiplication  of  rabbits  and  foxes  in  Australia, 
when  introduced  by  the  hand  of  man,  shows  what  might 
have  taken  place  in  this  country  had  not  impassable  barriers 
of  ocean  shut  them  out. 

160.  Subordinate  realms  or  provinces. — Each  of  these  great 
realms  may  be  indefinitely  subdivided  into  provinces  and 
sections,  for  there  is  no  end  to  the  possibility  of  analy- 
sis. No  school  district  has  exactly  the  same  animals  or 
plants  as  any  other,  as  finally  in  ultimate  analysis  we  find 
that  no  two  animals  or  plants  are  exactly  alike.  Shut  off 
one  pair  of  animals  from  the  others  of  its  species,  and  its 
descendants  will  differ  from  the  parent  stock.  This  differ- 


304  ANIMAL  LIFE 

ence  increases  with  time  and  with  distance  so  long  as  the 
separation  is  maintained.  Hence  new  species  and  new 
fauna  or  aggregations  of  species  are  produced  wherever 
free  diffusion  is  checked  by  any  kind  of  barrier. 

161.  Faunal  areas  of  the  sea. — In  like  manner,  we  may 
divide  the  oceans  into  faunal  areas  or  zones,  according  to 
the  distribution  of  its  animals.  For  this  purpose  the  fishes 
probably  furnish  the  best  indications,  although  results  very 
similar  are  obtained  when  we  consider  the  mollusks  or  the 
Crustacea.  The  fresh-water  fishes  are  not  considered  here, 
as  in  regard  to  their  faunal  areas  they  agree  with  the  land 
animals  of  the  same  regions.  Perhaps  the  most  important 
basis  for  primary  divisions  is  found  in  the  separation  from 
the  localized  shore-fishes  of  the  cosmopolitan  pelagic  species, 
and  the  scarcely  less  widely  distributed  bassalian  species  or 
fishes  of  the  deep  sea. 

The  pelagic  fishes  are  those  which  inhabit  the  open  sea, 
swimming  near  the  surface,  and  often  in  great  schools. 
Such  forms  are  mainly  confined  to  the  warmer  waters. 
They  are  for  the  most  part  predatory  fishes,  strong  swim- 
mers, and  many  of  the  species  are  found  in  all  warm  seas. 
Most  species  have  special  homing  waters,  to  which  they 
repair  in  the  spawning  season.  Often  there  will  be  special 
regions  to  which  they  never  resort,  either  for  breeding  or 
for  food.  At  other  times  a  certain  species  will  appear  in 
numbers  in  regions  where  it  has  hitherto  been  unknown. 
For  example,  the  frigate-mackerel  (Auxis  thazard),  homing 
in  the  East  Indies  and  the  Mediterranean,  appeared  in 
great  numbers  in  1880  off  the  coast  of  New  England.  Typ- 
ical pelagic  fishes  are  the  mackerel,  tunny,  dolphin,  flying- 
fish,  opah,  and  some  species  of  shark.  This  group  shades 
off  by  degrees  into  the  ordinary  shore-fish,  some  being  partly 
pelagic,  venturing  out  for  short  distances,  and  some  are 
pelagic  for  part  of  the  year  only.  To  the  free-swimming 
forms  of  classes  of  animals  lower  than  fishes,  found  in  the 
open  ocean,  the  name  Plankton  is  applied. 


GEOGRAPHICAL  DISTRIBUTION  OF  ANIMALS      305 


The  lassalian  fauna,  or  deep-sea  fauna,  is  composed  of 
species  inhabiting  great  depths  (2,500  feet  to  25,000  feet) 
in  the  sea.  At  a  short  distance  below  the  surface  the 
change  in  temperature  from  day  to  night  is  no  longer  felt. 
At  a  still  lower  depth  there  is  no  difference  between  winter 
and  summer,  and  still  lower  none  between  day  and  night. 
The  bassalian  fishes  in- 
habit a  region  of  great 
cold  and  inky  darkness. 
Their  bodies  are  subjected 
to  great  pressure,  and  the 
conditions  of  life  are  prac- 
tically unvarying.  There 
is  therefore  among  them 
no  migration,  no  seasonal 
change,  no  spawning  sea- 
son fixed  by  outside  con- 
ditions, and  no  need  of 
adaptation  to  varying  en- 
vironment. As  a  result,  all 
are  uniform  indigo-black 
in  color,  and  all  show  more 
or  less  degeneration  in 
those  characters  associated 
with  ordinary  environ- 
ment. Their  bodies  are 
elongate,  from  the  lack  of 
specialization  in  the  ver- 
tebrae. The  flesh,  being 
held  in  place  by  the  great 

pressure  of  the  water,  is  soft  and  fragile.  The  organs  of 
touch  are  often  highly  developed.  The  eye  is  either  exces- 
sively large,  as  if  to  catch  the  slightest  ray  of  light,  or  else 
it  is  undeveloped,  as  if  the  fish  had  abandoned  the  effort 
to  see.  In  many  cases  luminous  spots  or  lanterns  are  de- 
veloped by  which  the  fish  may  see  to  guide  his  way  in  the 
21 


FIG.  180.— A  crinoid  (Rhizocrinm  loxoten- 
sis).  A  deep-sea  animal  which  lives, 
fixed  plant  like,  at  the  bottom  of  the 
ocean. 


306  ANIMAL  LIFE 

sea,  and  in  some  forms  these  shining  appendages  are  highly 
developed.  In  one  form  (^Ethoprora)  a  luminous  body  cov- 
ers the  end  of  the  nose,  like  the  head-light  of  an  engine. 
In  another  (Ipnops)  the  two  eyes  themselves  are  flattened 
out,  covering  the  whole  top  of  the  head,  and  are  luminous 
in  life.  Many  of  these  species  have  excessively  large  teeth, 
and  some  have  been  known  to  swallow  animals  actually 
larger  than  themselves.  Those  which  have  lantern-like 
spots  have  always  large  eyes. 

The  deep-sea  fishes,  however  fantastic,  have  all  near  rel- 
atives among  the  shore  forms.  Most  of  them  are  degener- 
ate representatives  of  well-known  species — for  example,  of 
eels,  cod,  smelt,  grenadiers,  sculpin,  and  flounders.  The 
deep-sea  crustaceans  and  mollusks  are  similarly  related  to 
shore  forms. 

The  third  great  subdivision  of  marine  animals  is  the 
littoral  or  shore  group,  those  living  in  water  of  moderate 
depth,  never  venturing  far  into  the  open  sea  either  at  the 
surface  or  in  the  depths.  This  group  shades  into  both 
the  preceding.  The  individuals  of  some  of  the  species  are 
excessively  local,  remaining  their  life  long  in  tide  pools  or 
coral  reefs  or  piles  of  rock.  Others  venture  far  from  home, 
and  might  well  be  classed  as  pelagic.  Still  others  ascend 
rivers  either  to  spawn  (anadromous,  as  the  salmon,  shad, 
and  striped  bass),  or  for  purposes  of  feeding,  as  the  robalo, 
oorvina,  and  other  shore-fishes  of  the  tropics.  Some  live 
among  rocks  alone,  some  in  sea-weed,  some  on  sandy  shores, 
some  in  the  surf,  and  some  only  in  sheltered  lagoons.  In 
all  seas  there  are  fishes  and  other  marine  animals,  and 
each  creature  haunts  the  places  for  which  it  is  fitted. 


CLASSIFICATION  OF  ANIMALS* 

In  this  diagram  of  classification  every  animal  referred  to  in  this  book,  either  by  its 
vernacular  or  its  scientific  name,  is  assigned  to  its  proper  class  and  branch.  Of  the 
species  mentioned  by  their  scientific  names,  only  the  genus  name  is  given  in  this  list. 


KINGDOM  ANIMALIA 

BRANCH  I.    PROTOZOA 

CLASS  I.    Rhizop  oda. 

Amce'ba,  Globigeri'nae,  Radiola'ria. 
CLASS  II.    Mycetozo'a. 
CLASS  III.    MastigSph'ora. 

Volvocm'eae,  Go'nium,  Pandori'na,  Eudorl'na,  Vol'vox. 
CLASS  IV.     Sporozo  a. 

Gregarl  'na. 
CLASS  V.    Infuso'ria. 

Param&'ciiim,  Vorticel'la. 

BRANCH  II.    PORlF'ERA 

CLASS  I.    Porifera. 

Sponges,    Calcolyri  thus,    Prophyse'ma,    SpongU'la,    Spon'gia, 
Cll'ona. 

BRANCH  III.    CCELfiN'TERA'TA   (se-lgn-te-ra'-ta) 

CLASS  I.    Hydroz5'a. 

Hy'dra,  Euco'pe,  Siphonoph'ora,  Physoph'ora,  Obe'lia,  sea-anem'- 
one,  pol'yp,  Physa'lia,  Parapdg 'urus. 
CLASS  II.    Scyphozo'a  (sl-fo-zo'-a). 

Jelly-fish,  Llz'zia. 
CLASS  III.    Actinozo'a. 

CSr'als,  MeM'dium. 
CLASS  IV.    Ct^nbph'ora  (ten-oph'-o-ra). 

*  The  arrangement  of  branches  (or  phyla)  and  classes  here  used  is  that  adopted  in 
Parker  and  HasweH's  Text-Book  of  Zoology  (1897). 

307 


308  ANIMAL  LIFE 


BRANCH  IV.    PLATYHELMlN'THES 

CLASS  I.    Turbella'ria. 

Piano!  ria,. 

CLASS  II.    Tremato'da. 
CLASS  III.    CestS'da. 

Tape-worm,  Tce'nia,  Llg'ula,  flat-worm. 
APPENDIX  TO  PLATYHELMINTHES — CLASS  Nemertin  ea. 


BRANCH  V.    NEMATHELMlN'THES 

CLASS  I.    Nemato  da. 

Syn'gamus,    round-worm,    Trfchl'na,    Bothrioceph' alus,    pup- 
worm,  Unctna'ria. 
CLASS  II.    Acanthoceph  ala. 
CLASS  III.    Chaetbg'natha  (ke-tog'-na-tha). 

BRANCH  VI.    TROCHELMlN'THES 

CLASS  I.    Rotif'era. 

Rotato'ria. 

CLASS  II.    Dlnophi  lea. 
CLASS  III.    Gastrbt'richa. 

BRANCH  VII.    M6LLDSCOI'DA 

CLASS  I.    P61yzo  a. 
CLASS  II.    FhSrS'nida. 
CLASS  III.    Brachibp'oda. 

BRANCH  VIII.    ECHfNODER'MATA 

CLASS  I.    Asteroi  dea. 

Starfish. 

CLASS  II.    Ophiuroi'dea. 
CLASS  III.    Echinoi'dea. 

Sea-urchin. 
CLASS  IV.    Hblothuroi'dea. 

Sea-cucumber. 
CLASS  V.    Crinoi'dea. 

Crinoid,  Rhizocri 'nus. 
CLASS  VI.    Oystoi'dea. 
CLASS  VII.    Blastoi'dea. 


CLASSIFICATION  OF  ANIMALS  309 

BRANCH  IX.    ANNULA'TA 

CLASS  I.    ChaetSp'oda  (ke-t5p'-o-da). 

Earth-worm. 

APPENDIX  TO  THE  CH^TOPODA — CLASS  Myzostom  ida. 
CLASS  II.     Gephyre  a  (jef-e-re'-a). 
CLASS  III.    Archi-annelida. 
CLASS  IV.    Hirudin'ea. 

BRANCH  X.    ARTHRftP'ODA 

CLASS  I.   Crusta'cea. 

Lobster,  cray-fish,  crab,  barnacle,  Le'pas,  hermit-crab,  Pag'urus, 
pea-crab,  PmnotMres,  Epizoan'thus,  fish-lice,  whale-lice,  Saccu- 
ll'na,  Lernceo'cera,  prawn,  Pene  us. 

APPENDIX  TO  CRUSTACEA— CLASS  Trilobi  ta. 

CLASS  II.    Onychbph'ora. 

CLASS  III.    Myriap'oda.      . 
Cen'tiped. 

CLASS  IV.    InsSc'ta. 

Water-beetle,  water-bug,  canker-worm  moth,  bee,  white  ant, 
cockroach,  mosquito,  weevil,  grasshopper,  caterpillar,  butterfly, 
katydid,  beetle,  Dip'tera,  Lepidop'tera,  monarch  butterfly,  Ano'sia, 
Cu'lex,  Melari oplus,  May-fly,  locust,  cottony-cushion  scale,  Ice'rya, 
lady-bird,  Vedalia,  praying-horse,  Man'tis,  Ser'phus,  Cecro'pia, 
gall  insect,  An'dricus,  mole-cricket,  Gryllotal' pa,  Hydroph'ilus, 
Prlo'nus,  Campono 'tus,  plant-lice,  Aph'idae,  Coc'cidae,  Aphis-lion, 
ant,  Ec'iton.  termite,  bumble-bee,  carpenter-bee,  Andre'na,  Halic'- 
tus,  yellow- jacket,  hornet,  Ves'pa,  wasp,  At' ta,  bird-lice,  Malloph'- 
aga,  flea,  louse,  Pedic'ulus,  Lipeu'rus,  Hymenop 'tera,  ichneumon 
fly,  Thales'sa,  horn-tail,  Tre'mex,  PoUs'tes,  Stylop'idae,  Sty'lops, 
red  orange-scale,  toad-bug,  Gal'gulus,  inch- worm,  span-worm, 
geometrid,  walking-stick,  Diapherom'era,  Phyl'lium,  meadow 
brown,  Orap'ta,  Kal'lima,  sphinx-moth,  tomato-worm,  Phlege- 
tJion'tius,  puss-moth,  Ceru'ra,  viceroy  butterfly,  Basilar' chia,  Da- 
na'idae,  Helicon'idas,  Pier'idae,  Papnion'idae,  Syr'phidae,  flower-flies, 
tree-hopper,  MembrSc'idae,  Hemip'tera,  Sau'ba  (saw'-ba),  carrion- 
beetle,  Callosd'mia,  prome'thea,  cricket,  cica'da,  dragon-fly,  Cynip'- 
idae,  Hessian-fly,  mud-dauber,  Lerema,  Eryrinis,  skipper  butterfly, 
Schistocer'ca. 

CLASS  V.    Arach'nida. 

Tardig'rada,  bear-animalcule,  scorpion,  Lycos'idae,  tick,  itch- 
mite,  Sarcop'tes,  spider,  trap-door  spider,  turret-spider,  Ctent'za. 


310  ANIMAL  LIFE 

BRANCH  XI.    MtiLLtJS'CA 

CLASS  I.    Felecyp'oda. 

Clam,  pond-mussel,  Ll'ma. 
CLASS  II.    Amphineu'ra. 
CLASS  III.    GastrSp  oda. 

Pond-snail,  Lymnae'us,  whelk. 

APPENDIX  TO  THE  GASTROPODA — CLASS  Scaphbp'oda  AND  Rho  dope. 
CLASS  IV.    CephalSp'oda. 

BRANCH  XII.    CHORDA'TA 

SUB-BRANCH  I.    AdelochSr'da.    CLASS  Adeloohorda. 

SUB-BRANCH  II.    Urochor'da.    CLASS  Urochorda. 
Sea-squirts,  Tunica'ta. 

SUB-BRANCH  III.    Vertebra  ta.     DIVISION  A.  Acra'nia.    CLASS  Acra- 
nia.    DIVISION  B.  Crania  ta. 

CLASS  I.     Cyclostbniata. 

CLASS  II.    Pis'ces  (pis'-sez). 

Codfish,  sculpin,  skate,  lady-fish,  Al'lula,  sword-fish,  Xtph'ias, 
flounder,  Platoph'rys,  Salanx,  Cot'tus,  blob,  miller's- thumb, 
conger-eel,  Rem'ora,  Exocm'tus,  flying-fish,  Cypselu'rus  (sip-se-lu - 
rus),  deep-sea  angler,  lantern-fish,  Corynol ophus,  Eclitos'toma, 
^thoph'ora,  nokee,  scorpion-fish,  Emmy drfoh' thy s,  mad-tomr 
Schilbeodes,  cat-fish,  horned  pout,  toad-fish,  sting-ray,  globe-fish, 
porcupine-fish,  torpedo,  electric  eel,  electric  cat-fish,  star-gazer,  elec- 
tric ray,  Urol'ophus,  Dl'odon,  Narci'ne,  Ra'ja,  black-fish,  mud-fish, 
trout,  Sal'mo,  chub,  horned  dace,  Echeneididae  Amphiprion,  No'- 
meus,  hag-fish,  Myxl'ne,  Heptatre'ma,  Polistotre'ma,  lamprey, 
Oligocot'tus,  mouse-fish,  lava-fish,  Pterophryne,  pipe-fish,  Phyllop'- 
teryx,  anglers,  Lo'phius,  Antenna' rius,  Ceratiide,  minnow,  mack- 
erel, sucker,  salmon,  shad,  alewife,  sturgeon,  striped  bass,  qum'nat, 
eel,  sun-fish,  stickle-back,  carp,  cutlass-fish,  rainbow  darter,  chan- 
nel-cat, Au'xis,  tunny,  dolphin,  opah,  shark,  ^EtJiopro'ra,  Ip'nops, 
cod,  smelt,  grenadier,  rob'alo,  corvl'na,  CJiologas' ter,  TyphUch' thys, 
blind-fish. 

CLASS  III.    Amphibia 

Toad,  frog,  salamander,  tree-frog,  Hy'la. 

CLASS  IV.    Reptil'ia. 

Tortoise,  snake,  horned  toad,  Phrynoso'ma,  rattlesnake,  lizards, 
An'olis,  chameleon,  Gila  monster,  Heloder'ma,  JZlaps,  coralil'los, 
Lamproptt'tis,  Oscedla,  alligator. 


CLASSIFICATION  OF  ANIMALS  3H 

CLASS  V.    A'ves. 

Bird-of-paradise,  peacock,  pheasant,  robin,  pigeon,  chicken,  eagle, 
vulture,  guil'lemot  (gil'-e-mot),  murre  (miir),  auk,  ful'mar,  peVrel, 
sparrow,  bluebird,  woodpecker,  owl,  Colum'ba,  pelican,  MelanSr'pes, 
cormorant,  meadow-lark,  warbler,  turkey,  blue  jay,  Aythya,  Cya- 
nocitta,  Uria,  cow-bird,  cuckoo,  parrot,  ptarmigan,  whippoor- 
will,  Antros' tomus,  gull,  tern,  fly-catcher,  bittern,  mocking-bird, 
shrike,  bobolink,  goose,  humming-bird,  oriole,  puffin,  tailor-bird, 
king-bird,  nightingale,  starling,  skylark,  passenger-pigeon,  ouzel 
(ooz'-el),  linnet,  tanager,  vireo,  wood-warbler,  Phalacro' corax, 
Troch'ilus,  Ormthot'omus,  PsalMp' arus. 

CLASS  VI.    Mamma'lia. 

Horse,  ram,  fur-seal,  rabbit,  cat,  ox,  tiger,  lion,  sheep,  elephant, 
whale,  bear,  wolf,  squirrel,  lem'ming,  fox,  dog,  weasel,  stoat,  rein- 
deer, otter,  ant-eater,  giraffe',  skunk,  porcupine,  hedgehog,  arma- 
dillo, Callorhl'nus,  sea-lion,  deer,  buffalo,  kangaroo,  Mac'ropus, 
duck-bill,  Mon'otreme,  monkey,  gopher,  elk,  bison,  prairie-dog, 
big-horn,  hare,  antelope,  black-tail  deer,  hound,  mole,  hyena,  mice, 
rodent,  woodchuck,  jack-rabbit,  Maca'cus,  Cercoptth' ecus,  beaver, 
wood-rat,  pocket-gopher,  coyo'te,  civet-cat,  flying-fox,  mon'goose, 
sea-cow,  Vul'pes,  walrus,  musk-ox,  ape,  rhmoc'eros,  hippopot'amus, 
leopard,  ass,  le'inur,  Trich! gchit*,  manatee,  Le'pus,  OdobcB'nus, 
Le'mur. 


GLOSSARY 

[Only  those  terms  are  defined  in  this  glossary  that  are  not  explained  in  the  text. 
In  the  case  of  the  terms  defined  or  explained  in  the  text,  reference  is  made  to  the 
number  of  the  paragraph  in  which  the  explanation  occurs.  The  pronunciation  of  the 
vernacular  and  scientific  names  of  the  animals  mentioned  in  the  text  is  given  in  the 
Classification.] 

Abomasum:  42. 

Adaptation :  67,  74. 

Albuminous  :  said  of  substances  containing  albumen, 

Alimen  tary  canal :  42. 

Alluring  coloration :  111. 

Altricial:  79. 

Altruistic  instinct:  129. 

Amoe'boid :  having  the  changing  form  of  an  Amreba. 

Anad  romous :  said  of  fishes  that  go  from  the  sea  up  rivers  to  lay  their 
eggs. 

Anatomy:  39. 

Animalcule :  an  animal  of  microscopic  smallness. 

Anten  nae  :  the  "  feelers,"  the  most  anterior  pair  of  appendages  of  in- 
sects and  insect-like  animals  ;  situated  on  the  head,  and  the  seat  of 
organs  of  special  sense. 

Anthropoid:  man-like. 

A'nus :  42. 

Appen'dix  vermifor  mis  :  82. 

Artificial  selection :  72. 

Assim  ilate  :  to  receive  food  and  transform  it  into  a  homogenous  part 
of  the  body  substance. 

At51T  :  a  ring-shaped  coral  island  nearly  or  quite  inclosing  a  lagoon. 

Atrophy  :  a  stoppage  of  the  growth  or  development  of  a  part  or  organ. 

Auditory  :  referring  to  the  sense  of  hearing. 

Aut6m  atism :  the  state  of  being  automatic ;  involuntary  action. 

313 


314  ANIMAL  LIFE 

Bassalian:  160. 

Biologist :  student  of  animals  and  plants. 

Blastoderm:  50. 

Blas'tula:  50. 

Budding :  the  process  of  reproduction  among  animals  in  which  a  small 
part  of  the  body  substance  of  an  animal  grows  out  from  the  sur- 
face, separates  from  the  parent,  and  develops  into  a  new  individual. 

Cae'cum  (se-kum) :  42. 

Carniv'orous :  flesh-eating. 

Cat  arrhine  :  nostril  downward ;  said  of  the  narrow-nosed  Old- World 

monkeys. 
Cell :  2. 
Cellulose :  a  peculiar  compound  insoluble  in  all  ordinary  solvents, 

forming  the  fundamental  material  of  the  structure  of  plants,  and 

also  contained  in  the  mantle  of  tunicates. 
Chi  tin:  57. 
ChlS'rophyll :  13. 

Chro'matophore  :  a  color-bearing  granule  or  sac. 
Chro  mosome :  2. 
Chrys  alis :  57. 
Chyle:  42. 
Cilia:  5. 
Cleavage:  50. 
C51on:  42. 
Commen  salism :  90. 
Com'munal:  83. 
Conjugation:  5. 
Contractile  vacuole :  a  vacuole  that  dilates  and  contracts  regularly, 

and  is  supposed  to  have  an  excretory  function. 
Cyst :  98. 
Cytoplasm:  2. 

Degeneration:  95. 
Development:  46. 
Differentiation :  the  setting  apart  of  special  organs  for  special  work ; 

progressive  change  from  general  to  special ;  specialization. 
Digestion :  the  process  of  dissolving  and  chemically  changing  food  so 

that  it  can  be  assimilated  by  the  blood  and  furnish  nutriment  to 

the  body. 

Dimor  phism :  24. 

Dlvertic'ulum :  a  blind  pouch  arising  from  another  larger  pouch.    44. 
Duode'num:  42. 


GLOSSARY  315 

Ec'toblast:  50. 
Ec'toderm:  20. 
Egg-cell:  20. 
Egois'tic  instinct :  129. 
EmbrybTogy:  39. 
Embrybn  ic  :  49. 
En'doblast:  50. 
En  do  derm  :   20. 

Environment :  an  organism's  surroundings  taken  collectively. 
Ex'cretory:  referring  to  excretion,  as  excretory  organs,  the  organs 
which  get  rid  of  waste  matter  in  the  animal  body. 

Fau'na  (fawna) :  157. 
Fertilized  egg :  20. 
Fission:  4. 
FlageTla:  13. 
Function:  37. 

Ganglion  (pi.  ganglia) :  a  nerve-center  composed  of  an  aggregation  of 

nerve-cells. 
GSs'trula:  50. 
Gem' mule:  20. 
Generalization:  41. 

Geologist :  student  of  the  structure  and  history  of  the  earth. 
Gregarious:  87. 
Growth:  46. 

Habit:  139. 

Herbiv'orous :  plant-eating. 

Heredity:  54. 

Hermaphroditic:  35. 

Hlberna  tion :  passing  the  winter  in  a  death-like  sleep. 

Homoge  neous :  of  the  same  composition  or  structure  throughout. 

ileum:  42. 

Inorganic :  not  being  nor  having  been  a  living  organism  ;  not  organic. 

Insectiv'orous :  insect-eating. 

Instinct:  128. 

Intellect:  139. 

Intercellular :  outside  of  and  between  the  cells. 

Jejunum:  42. 


316  ANIMAL  LIFE 

Lagoon' :  a  pool  or  lake ;  the  still  water  inclosed  within  an  atoll. 

Larva:  57. 

LSpidBp  terous  :  referring  to  the  Lepidoptera,  or  moths  and  butterflies, 

Littoral:  160. 

Lu  men :  the  cavity  of  a  tubular  organ.    44. 

Medu'sa:  24. 

Megalops:  59. 

Mes'oderm:  20. 

Metab  olism :  the  act  or  process  by  which  dead  food  is  built  up  into 
living  matter,  and  living  matter  is  broken  down  into  simpler  prod- 
ucts within  a  cell  or  organism. 

Metamorphosis:  56. 

Migration:  70. 

Millimeter :  about  one  twenty-fifth  of  an  inch ;  a  term  used  in  the 
metric  system  of  measure. 

Mimicry:  112. 

Mind:  140. 

Molt:  57. 

M5nSg  amous :  said  of  animals  in  which  a  male  mates  with  only  a 
single  female. 

Multiplication:  used  in  the  text  usually  synonymously  with  repro- 
duction. 

Myrmec8ph  ilous :  said  of  insects  which  are  found  inhabiting  the  nests 
of  ants. 

Natural  selection:  70. 

N5'tochord :  an  elastic  rod,  or  row  of  cells,  formed  in  the  early  embryo 
of  chordate  animals  (including  all  the  vertebrates  and  some  others), 
which  lies  below  the  dorsal  nervous  tube  and  above  the  ventral  ali- 
mentary tube. 

Nu'cleus  (pi.  nuclei) :  2. 

CEsSph'agus:  42. 

Olfactory :  referring  to  the  sense  of  smell. 
Oma'sum:  42. 
Organ:  37. 

Organic :  referring  to  the  matter  of  which  animals  and  plants  are  com- 
posed. 

Organism :  a  living  being,  plant  or  animal. 
Orientation:  95. 
Otolith:  121. 


GLOSSARY  317 

Papilla  (pi.  papillae) :  a  small  nipple-like  process,  as  the  papillae  of  the 
skin  or  tongue. 

Parasite:  93. 

Parthenogenesis:  35. 

Pelagic :  inhabiting  the  surface  of  mid-ocean.     160. 

Pharynx:  42. 

FhysibTogy:  39. 

Plat  yrrhine :  broad-nosed  ;  said  of  the  New- World  monkeys. 

Plu'teus:  59. 

Folyg'amous :  said  of  animals  in  which  a  single  male  mates  with  sev- 
eral females :  35. 

Pblymor'phism :  24. 

Polyp:  21. 

Post-embryonic :  49 

Praeco  cial  (pre-co'-shal) :  79. 

Predatory:  feeding  on  other  animals. 

Propolis:  84. 

Protective  resemblance:  107. 

Protoplasm:  2. 

PrSventric'ulus :  42. 

Pseu'dopod :  4. 

Psy'chic:  140. 

Pupa:  57. 

Reason:  139. 

Recognition  mark :  77,  115. 
Rec'tum:  42. 
Reflex  action :  127. 
Reproduction:  67. 
Respiration:  4. 
Retic'ulum:  42. 
Retina:  123. 
Ru'men :  42. 
Ruminant:  42. 

Saliva:  42. 

Sensation:  67. 

Sensorium :  126. 

Silica :  the  mineral  of  which  quartz,  sand,  flint,  etc.,  are  composed. 

Spawn,  v. :  to  lay  eggs. 

Specialization:  41. 

Species:  151. 


318  ANIMAL  LIFE 

Sperm  cell :  20. 

Spiracle :  one  of  the  breathing  openings  of  an  insect  situated  on  the 

side  of  the  abdomen  or  thorax. 
Spon'gin :  20. 
Stimulus  (pi.  stimuli) :  that  which  excites  action  in  plant  or  animal 

tissue. 

Strata  :  layers,  usually  said  of  rocks. 
Stridula  tion :  122. 
Sub-species  :  151. 
Symbiosis:  90. 

Tactile  :  referring  to  the  sense  of  touch.    118. 

Tadpole:  58. 

Ten'tacle :  a  protruding  flexible  process  or  appendage,  usually  of  the 

head  of  invertebrate  animals,  being  used  as  an  organ  of  touch, 

prehension,  or  motion. 
Termitoph  ilous  :  said  of  insects  inhabiting  the  nests  of  termites. 

Vac'uole :  a  minute  cavity  containing  air,  water,  or  a  chemical  secre-. 

tion  of  the  protoplasm,  found  in  an  organ,  tissue,  or  cell. 
Vestigial :  82. 
Vis'cera :  the  organs  in  the  great  cavities  of  the  body,  commonly  used 

for  the  organs  in  the  abdominal  cavity. 

Yolk  (yok) :  48. 

Zoea:  59. 

ZSogebg'raphy :  147. 

Zo'oid :  one  of  the  more  or  less  independent  members  of  a  colonial  or 

compound  organism. 
Zoologist :  a  student  of  animals. 


INDEX 


Abomasum,  68. 

Actinocephalus  otigacanthus  (ill.), 
14. 

Adaptations,  113,  123;  classifica- 
tion of,  123  ;  concerned  with  sur- 
roundings, 143 ;  degree  of  struc- 
tural change  in,  146  ;  for  defense 
of  young,  137 ;  for  rivalry,  135 ; 
for  self-defense,  128 ;  for  secur- 
ing food,  125 ;  origin  of,  123. 

Agassiz's  cave-fish  (illus.),  282. 

Albula  vulpes,  metamorphosis  of 
(illus.),  98. 

Alimentary  canal,  66  ;  of  cock- 
roach (illus.),  73  ;  of  earthworm 
(illus.),  71 ;  of  flatworm  (illus.), 
70;  of  Holothurian  (illus.),  70; 
of  mussel  (illus.),  72  ;  of  Obelia 
(illus.),  69 ;  of  ox  (illus.),  67  ;  of 
Planaria  (illus.),  70;  of  sea- 
cucumber  (illus.),  70. 

Alligator  (illus.),  290. 

Alluring  coloration,  216. 

Alternation  of  generations,  42. 

Altricial,  140. 

Amoeba,  5;  multiplication  of  (ill.)  53. 

Ammba  polypodia  (illus.),  8. 

Anatomy,  64. 

Andrena,  nest  of  (illus.),  160. 

Andricus  californicus,  galls  of 
(illus.),  143. 


Angler,  deep-sea  (illus.),  124. 

Animals,  life  of  simplest,  1 ;  many- 
celled,  2 ;  one-celled,  2  ;  slightly 
complex,  24. 

Anosia  plexippus,  metamorphosis 
of  (illus.),  92  ;  mimicked  by  Ba- 
silarchia  archippus  (illus.),  219. 

Antenna  of  cray-fish  (illus.),  233  ; 
of  leaf-eating  beetle  (illus.),  230. 

Antennae,  specialized,  of  prome- 
thea  moth  (illus.),  231. 

Antrostomus  vociferus  (illus.),  203. 

Ants  (illus.),  155. 

Anus,  68. 

Appearance,  terrifying,  212. 

Arctic  realm,  297. 

Area,  faunal,  296. 

Artificial  selection,  120. 

Auditory  organ  of  cray-fish  (illus.), 
233;  of  cricket  (illus.),  234;  of 
grasshopper  (illus.),  234 ;  of  mol- 
lusk  (illus.),  233;  of  mosquito 
(illus.),  235. 

Auditory  organs,  232. 

Australian  realm,  303. 

Aythya  (illus.),  137. 

Barbadoes  earth,  19. 

Barnacle,  adult  and  larva  (illus.), 
195;  metamorphosis  of  (illus.), 
101.  319 


320 


ANIMAL  LIFE 


Barrier,  mountains  a,  to  distribu- 
tion, 293 ;  sea  a,  to  distribution, 
288 ;  temperature  a,  to  distribu- 
tion, 290. 

Barriers  affecting  fresh-water  ani- 
mals, 294 ;  effect  of,  283 ;  species 
debarred  by,  274;  to  distribu- 
tion, character  of,  288. 

Basilarchia  archippus  mimicking 
Anosia  plexippus  (illus.),  219. 

Bassalian  fauna,  305. 

Beavers,  nest  of  (illus.),  269. 

Beetle,  larva  of  (illus.),  146. 

Beetles,  lady-bird  (illus.),  214. 

Bird,  egg  of  (illus.),  79. 

Bird-louse  (illus.),  188. 

Bird  of  paradise  (illus.),  58. 

Birds,  nest-making  habits  of,  264. 

Birth,  78. 

Bittern,  nestlings  of  (illus.),  246, 
247. 

Blastoderm,  82. 

Blastula,  82. 

Blue  jay  (illus.),  138. 

Brain,  241. 

Budding,  13. 

Bumble-bee,  159,  (illus.),  161. 

Bush-tit,  nest  of  California  (illus.), 
270. 

Butterfly,  egg  of  (illus.),  79 ;  mon- 
arch, metamorphosis  of  (illus.), 
92. 

Caecum,  68. 

Calcolynthus  porimigenius  (illus.), 
33. 

Calf,  taste  buds  of  (illus.),  229. 

CallorTiinus  alascanus  (illus.), 
136. 

Camponotus  (illus.),  155. 

Canadian  skipper  butterfly,  distri- 
bution of  (illus.),  273. 


Canal,  alimentary,  66. 
Cankerworm-moth  (illus.),  59. 
Care  of  young  of  mammals,  268. 
Carpenter-bee,  nest  of  (illus.),  160. 
Caterpillar  parasitized  (illus.),  189, 

(illus.),  190. 

Cave  blind-fish  (illus.),  282. 
Ceanothus  (illus.),  1.41. 
Cell,  animal,  2 ;  egg,  21,  56 ;  plant, 

2 ;  products,  3 ;  wall,  3. 
Cells,  brood,  of  honey-bee  (illus.), 

152;   nerve,  240;   reproductive, 

29,  55. 

Cellulose,  24,  27. 
Centiped  (illus.),  130. 
Cerura,  larva  of  (illus.),  216. 
Chalk,  18. 
Chick,  embryonic  stages  of  (illus.), 

87. 

Chitin,  91. 
Chlorophyll,  24. 

Chologaster  agassizi  (illus.),  282. 
Chologaster  avetus  (illus.),  282. 
Chromatophore,  24. 
Chromosome,  3. 
Chrysalid    of    butterfly,    showing 

protective    resemblance    (illus.), 

206. 

Chrysalis,  93. 
Chyle,  68. 
Cilia,  9. 
Cleavage,  82. 
Climate,  influencing  distribution, 

291 ;  instincts  of,  248. 
Clouded  skipper  butterfly,  distri- 
bution of  (illus.),  273. 
Coccidium  oviforme  (illus.),  14. 
Cockroach,    alimentary    canal    of 

(illus.),  73;  egg  case  of  (illus.), 

140. 
Cocoon  of  Cecropia  moth  (illus.), 

141. 


INDEX 


321 


Colon,  68. 

Colonial  jelly-fishes,  45 ;  Protozoa, 

24. 

Colony,  31. 
Color,  222. 

Coloration,  alluring,  216. 
Colors,  warning,  212. 
Commensalism,  172,  173. 
Communal  life,   168;    advantages 

of,  170. 

Communities,  animal,  149. 
Conditions,    primary,    of    animal 

life,  106. 

Conjugating  cells,  28. 
Conjugation,  11,  27,  55. 
Contractile  vacuole,  10. 
Coral,  brain,  45 ;     island   (illus.), 
44 ;  organ-pipe  (illus.),  45 ;  red, 
45. 

Corals,  37-43. 
Corynolophus    reinhardti    (illus.), 

124. 

Cottony  cushion  scale  (illus.),  142. 
Courtship,  instincts  of,  248. 
Crab,  metamorphosis  of  (illus.),  97 ; 

with  sea-anemone  (illus.),  177. 
Cray-fish,  auditory  organ  of  (illus.), 

233. 
Cricket,  auditory  organ  of  (illus.), 

234. 

Cricket,  mole  (illus.),  146. 
Crinoid  (illus.),  305. 
Crop,  71. 

Crowd  of  animals,  114. 
Crustaceans,    adults    and     larvae 

(illus.),  195. 
Cteniza  californica,  nest  of  (illus.), 

261. 

Cyanocitta  cristata  (illus.),  138. 
Cycle,  life,  78. 
Cypselurus  (illus.),  131. 
Cytoplasm,  3. 
22 


Dead-leaf  butterfly  (illus.),  211. 

Death,  103. 

Deep-sea  angler  (illus.),  124. 

Deer,  horns  of  (illus.),  148. 

Defense  of  the  young,  137. 

Degeneration,  causes  of,  197,  198; 
human,  200 ;  through  quiescence, 
193. 

Desiccation,  104. 

Development,  78  :  continuity  of, 
83  ;  divergence  of,  84  ;  embry- 
onic, 80;  first  stages  in  (illus.), 
81 ;  laws  of,  86 ;  metamorphic, 
90;  of  flounder  (illus.),  100;  of 
locust  (illus.),  91 ;  of  vertebrates, 
(illus.),  87;  post-embryonic,  80; 
significance  of  facts  of,  89. 

Diapheromera  femorata  (illus.), 
209. 

Differentiation,  41 ;  of  structure, 
64. 

Dimorphism,  42 ;  sex,  58. 

Diodon  hystrix  (illus.),  134. 

Dismal  Swamp  fish  (illus.),  282. 

Distribution,  character  of  barriers, 
to,  288;  geographical,  272;  in- 
fluenced by  climate,  291;  laws 
of,  274;  modes  of,  296;  moun- 
tains a  barrier  to,  293 ;  of  Cana- 
dian Skipper  butterfly  (illus.), 
273 ;  of  clouded  Skipper  butter- 
fly (illus.),  273 ;  of  Erynnis  mani- 
toba  (illus.),  273;  sea  a  barrier 
to,  288;  temperature  a  barrier 
to,  290. 

Diverticula,  74. 

Division  of  labor,  22, 168. 

Dog,  pointer  (illus.),  256. 

Dragon-fly,  eye  of  (illus.),  239. 

Duck,  family  (illus.),  137. 

Duodenum,  68. 
Duration  of  life,  101. 


322 


ANIMAL  LIFE 


Earthworm,  alimentary  canal  of 
(illus.),  71. 

Ectoblast,  82. 

Ectoderm,  33. 

Egg  case  of  Californian  barn-door 
skate  (illus.),  140 ;  cockroach  (il- 
lus.), 140. 

Egg  cell,  21,  56. 

Egg,  fertilized,  35 ;  of  bird  (illus.), 
79;,  of  butterfly  (illus.),  79;  of 
fish  (illus.),  79;  of  katydid  (il- 
lus.), 79  ;  of  skate  (illus.),  79  ;  of 
toad  (illus.),  79. 

Electric  ray.  (illus.),  135. 

Embryology,  64. 

Embryonic  development,  80;  of 
the  pond  snail,  81. 

EmmydricJithys  vulcanus  (illus.), 
132. 

Endoblast,  82. 

Endoderm,  33. 

Environment,  instincts  of,  248. 

Epizoanthus  paguriphilus,  with 
sea-anemone  (illus.),  177. 

Erynnis  manitoba,  distribution  of 
(illus.),  273. 

Eucope  (illus.),  42. 

Eudorina,  27. 

Eudorina  elegans  (illus.),  28. 

Exocwtus  (illus.),  131. 

Eye  of  dragon-fly  (illus.),  239 ;  of 
jelly-fish  (illus.),  238. 

Fauna,  296;  bassalian,  305  ;  littoral, 
306 ;  pelagic,  304. 

Faunal  areas  of  the  sea,  304. 

Feeding  habit  of  Californian  wood- 
pecker (illus.),  128, 129  ;  ofMela- 
nerpes  formicivorus  bairdii 
(illus.),  128,  129;  instincts  of, 
244. 

Female,  57. 


Fish,  egg  of  (illus.),  79;  embry- 
onic stages  of  (illus.),  87  ;  -louse 
(illus.),  188. 

Fishes,  man-of-war  (illus.),  175; 
nest-making  habits  of,  264. 

Fission,  9 ;  binary,  54. 

Flagella,  25. 

Flagellata,  24. 

Flatworm,  alimentary  canal  of 
(illus.),  70. 

Flounder,  development  of  (illus.), 
100 ;  wide-eyed  (illus.),  100. 

Flying  fishes  (illus.),  131. 

Food,  adaptations  for  securing, 
125;  necessary  to  animal  life, 
106. 

Form,  primitive,  20. 

Fossil,  18. 

Fresh-water  animals,  barriers  af- 
fecting, 294. 

Function,  63. 

Fur  seal  (illus.),  136. 

Galapagos    Islands,     animals     of 

(illus.),  278;    locusts  of  (illus.), 

280. 
Gall,  giant,  of  white  oak  (illus.), 

143. 

Galls,  insect,  on  leaf  (illus.),  144. 
Gapes,  worm  which  causes,  60. 
Gastrula,  82. 
Gemmule,  35. 
Generalization,  66. 
Generation,  spontaneous,  51. 
Generations,  alternation  of,  51. 
Geographical  distribution,  272. 
Geometrical  larva  on  branch  (illus.), 

209. 
Gerrhonotus   scincicauda    (illus.), 

204. 

Giraffe  (illus.),  126. 
Gizzard,  71. 


INDEX 


323 


Globigerina-ooze,  18. 

Globigerinae,  16. 

Gonium,  25-30. 

Gonium  pectorale  (illus.),  25. 

Grasshopper,    auditory    organ    of 

(illus.),  234. 

Green-leaf  insect  (illus.),  210. 
Gregarina,  13,  182. 
Gregarina  polymorpha  (illus.),  14. 
Gregarinidas,  14. 
Gregariousness,  163. 
Growth,  78. 
Gryllotalpa  (illus.),  146. 

Habit,  251. 

Habitat,  relation  of  species  of,  283. 

Habits,  domestic,  257. 

Habits,  nest-making,  of  birds,  264 ; 
of  fishes,  264 ;  of  insects,  262  ;  of 
invertebrates,  258;  of  spiders, 
259 ;  of  vertebrates,  264. 

Hearing,  sense  of,  232. 

Heliosphc&ra  actinota  (illus.),  19. 

Heredity,  89. 

Hermaphroditism,  60. 

Hermit-crab,  with  the  sea-anemone 
(illus.),  176. 

Hibernation,  103. 

Hiving  honey-bees  (illus.),  154. 

Holothurian,  alimentary  canal  of 
(illus.),  70. 

Homes,  257. 

Honey-bee  (illus.),  150;  adult  and 
larva  (illus.),  83;  leg  of  (illus.), 
151 :  life  history  of,  149. 

Honey-bees,  hiving  a  swarm  of 
(illus.),  154. 

Host,  relation  of  parasite  to,  179. 

Human  degeneration,  200. 

Humming-bird,  nest  of  rufus 
(illus.),  265,  266. 

Hydra,  37. 


Hydra  vulgans  (illus.),  38. 
Hydrophilus  (illus.),  146. 
Hyla  regilla  (illus.),  145. 

Icerya  and  Vedalia,  121. 

Icerya  purchasi  (illus.),  142. 

Ileum,  68. 

Individual,  31. 

Indo- African  realm,  301. 

Inorganic  matter,  112. 

Insect  galls  on  leaf  (illus.),  144. 

Insects,  metamorphosis  of,  90;  nest- 
making  habits  of,  262 ;  parasitic, 
188. 

Instinct,  242. 

Instincts,  altruistic,  243 ;  classifi- 
cation of,  243  ;  concerned  with 
care  of  the  young,  250 ;  egoistic, 
243;  of  climate,  248;  of  court- 
ship, 248  ;  of  environment,  248 ; 
of  feeding,  244 ;  of  play,  247 ;  of 
reproduction,  249;  of  self-de- 
fense, 245  ;  variability  of,  251. 

Intellect,  254. 

Intestine,  68. 

Invertebrates,  nest-making  habits 
of,  258. 

Irritability,  8,  240. 

Island,  coral  (illus.),  44. 

Itch-mite  (illus.),  192. 

Jack-rabbits,    showing     variation 

(illus.),  281. 

Jay,  Canada  (illus.),  138. 
Jejunum,  68. 

Jelly-fish,  eye  of  (illus.),  238. 
Jelly-fishes,  37  ;  colonial,  45. 

Kallima  (illus.),  211. 
Kangaroo  (illus.),  139. 
Katydid,  egg  of  (illus.),  79. 
Lady-bird  beetles  (illus.),  214. 


324 


ANIMAL  LIFE 


Lady-fish,  metamorphosis  of  (illus.), 
98. 

Larva,  92;  of  the  mosquito,  93; 
of  butterfly  pupating  (illus.),  94 ; 
of  the  honey-bee,  152. 

Leaf -cutting  ant  mimicked  by  tree- 
hoppers  (illus.),  220. 

Lemur  (illus.),  302. 

Lemur  varius  (illus.),  302. 

Lemurian  realm,  302. 

Lepas,  adult  and  larva  (illus.),  195  ; 
metamorphosis  of  (illus.),  101. 

Lerema  accius,  distribution  of 
(illus.),  273. 

Lernceocera  (illus.),  188. 

Life  cycle,  78. 

Life,  communal,  168 ;  duration  of, 
101 ;  primitive,  21 ;  processes, 
21 ;  social,  149. 

Light,  influence  of,  on  animals, 
237. 

Lipeureus  densus  (illus.),  188. 

Littoral  fauna,  306. 

Lizard,  alligator  (illus.),  204. 

Lizzia  koellikeri,  eye  of  (illus.), 
238. 

Locust,  post-embryonic  develop- 
ment of  (illus.),  91. 

Locusts  of  Galapagos  Islands 
(illus.),  280. 

Louse,  sucking  (illus.),  188. 

Macropus  rufus,  139. 

Mad  Tom  (illus.),  132. 

Male,  57. 

Mammals,  care  of  young  of,  268. 

Manatee  (illus.),  277. 

Man-of-war,     Portuguese    (illus.), 

175. 

Mantis  (illus.),  127. 
Many-celled  animal,  2. 
Marine  Protozoa,  15. 


Marks,  recognition,  22,  129,  223. 

Medusae,  41. 

Megalops,  97. 

Melanerpes  formicivorus  bairdii, 
feeding  habit  of  (illus.),  128, 129. 

Membracidae  mimicking  Sauba 
ant  (illus.),  220. 

Mesoderm,  33. 

Metamorphosis,  90  ;  of  Albula  vul- 
pes  (illus.),  98 ;  of  Anosia  plex- 
ippus  (illus.),  92;  of  barnacle 
(illus.),  101 ;  of  butterfly  (illus.), 
92;  of  crab  (illus.),  97;  of  in- 
sects, 90;  of  lady-fish  (illus.), 
98;  of  Lepas  (illus.),  101;  of 
mosquito  (illus.),  93  ;  of  sea-ur- 
chin (illus.),  96;  of  sword-fish 
(illus.),  99;  of  toad,  94,  (illus.), 
95 ;  of  Xiphias  gladius  (illus.), 
99. 

Metazoa,  32. 

Metridium  dianthus  (illus.),  43. 

Micro-organism,  16. 

Migration  of  lemming,  118  ;  of  lo- 
cust, 118. 

Mimickry,  218. 

Mind,  255. 

Mining-bee,  nest  of  (illus.),  160. 

Molt,  91. 

Monarch  butterfly  (illus.),  219  ; 
mimicked  by  Viceroy  butterfly 
(illus.),  219. 

Monogamy,  135. 

Mosquito,  auditor  organ  of  (illus.), 
235  ;  head  of  (illus.),  127 ;  meta- 
morphosis of  (illus.),  93;  young 
stages  of  (illus.),  147. 

Moth,  cankerworm  (illus.),  59. 

Mountains  as  barriers  to  distribu- 
tion, 293. 

Mouth  parts  of  mosquito  (illus.), 
127. 


INDEX 


325 


Mouse-fish  in  gulf-weed  (illus),  208. 
Mt.  Orizaba  (illus.),  300. 
Multiplication,    50 ;    of    animals, 

114 ;    simplest    method  of,   53 ; 

slightly  complex  meihods  of,  54. 
Murres,  Pallas's  (illus.).  165. 
Mussel,  alimentary  canal  of  (illus.), 

72. 
Mutual  aid,  163. 

Narcine  brasiliensis  (illus.),  135. 

Natural  selection,  117. 

Neotropical  realm,  301. 

Nerve  cells,  240. 

Nerve  fibers,  240. 

Nest-making  habits  of  birds,  264 ; 
of  fishes,  264;  of  insects,  262; 
of  invertebrates,  258 ;  of  spiders, 
259 ;  of  vertebrates,  264. 

Nest  of  Baltimore  oriole  (illus.), 
267;  of  beavers  (illus.),  269;  of 
Californian  bush-tit  (illus.),  270 ; 
of  Cleniza  californica  (illus.), 
261 ;  of  pocket  -  gopher  (illus.), 
271;  of  rufus  humming-bird 
(illus.),  265,  266;  of  tailor-bird 
(illus.),  268 ;  of  trap-door  spider 
(illus.),  261;  of  turret  -  spider 
(illus.),  262. 

Nokee  (illus.),  132. 

Nomeus  gronovii  (illus.),  175. 

North  Temperate  realm,  299. 

Nuclear  membrane,  3. 

Nucleus,  3. 

Obelia,  alimentary  sac  of  (illus.), 

69. 

CEsophagus,  67. 
Omasum,  67. 
One-celled  animals,  2. 
Organ,  63;  auditory,  of  cray-fish 

(illus.),  233;  of  cricket  (illus.), 


234 ;  of  grasshopper  (illus.),  234 ; 
of  mosquito  (illus.),  235 ;  of  mol- 
lusk  (illus.),  233. 

Organic  matter,  112. 

Organs,  auditory,  232;  of  smell, 
229;  of  sound-making,  235;  of 
taste,  228;  of  touch,  226;  spe- 
cialization of,  66 ;  vestigial,  147. 

Oriole,  Baltimore,  nest  of  (illus.), 
267. 

Ornithotomous  sutorius,  nest  of, 
268. 

Osprey  Falls  (illus.),  285. 

Otolith,  232. 

Ox,  alimentary  canal  of  (illus.)  67. 

Oxygen,  necessary  to  animal  life, 
107. 

Pagurus  (illus.),  176. 
Pandorina,  26. 
Pandorina  sp.  (illus.),  26. 
Pandorina  morum  (illus.),  27. 
Papilio,  chrysalid  of  (illue.),  205. 
Papilla,   tactile,   of    skin  of  man 

(illus.),  227. 
Papilla?,  67. 
Paramcecium,  9. 
Paramcecium  aurelia  (illus.),  10. 
Paramcecium  caudatum  (illus.),  1 1. 
Paramcecium  plutorinum  (illus.), 

11. 
Parasite  of  caterpillar  (illus.),  190 ; 

relation  to  host,  179. 
Parasites,  simple  structure  of,  181. 
Parasitism,  179 ;  kinds  of,  180. 
Parthenogenesis,  60. 
Patagonian  realm,  302. 
Pediculus  (illus.),  188. 
Pelagic  fauna,  304. 
Pelican,  brown  (illus.),  125. 
Peneus,  adult  and    larva  (illus.), 

195. 


326 


ANIMAL   LIFE 


Pharynx,  71. 

Phlegethontius  Carolina,  larva  of 

(illus.),  215. 
Phrynosoma     Uainvillei     (illus.), 

131. 

PJiyllium  (illus.),  210. 
Phyllopteryx  (illus.),  212. 
Physalia  (illus.),  175. 
Physiology,  64. 
Physoptora  (illus.),  47. 
Pocket-gopher,  nest  of  (illus.),  271. 
Pointer  dog  (illus.),  256. 
Polistes,    parasitized   by    Stylops 

(illus.),  192. 
Polygamy,  59. 
Polymorphism,  42. 
Polyps,  37. 

Polystomella  strigillata  (illus.),  17. 
Porcupine-fish  (illus.),  134. 
Post-embryonic  development,  80. 
Pigeon  horn-tail  (illus.),  191. 
Pipe-fish  (illus.),  212. 
Planaria,    alimentary    canal     of 

(illus.),  70. 
Plankton,  304. 
Plants,  difference  between  animals 

and,  111. 

Platophrys  lunatus  (illus.),  100. 
Play,  instincts  of,  247. 
Pluteus,  96. 
Praecocial,  140. 

Prawn,  adult  and  larva  (illus.),  195. 
Praying-horse  (illus.),  127. 
Pressure,  a    condition  of  animal 

life,  109. 

Primitive  form,  20. 
Primitive  life,  21. 
Prionus,  larva  of  (illus.),  146. 
Processes,  life,  21. 
Promethea  moth  (illus.),  231. 
Prophysema  primordiale    (illus.), 

34. 


Protective  resemblance,  201. 

Protoplasm,  3;  chemical  constitu- 
tion of,  4 ;  physical  constitution 
of,  4. 

Protozoa,  1 ;  colonial,  24 ;  marine, 
15. 

Psaltriparus  minimus,  nest  of 
(illus.),  270. 

Pseudopod,  5. 

Pterophryne  histrio  in  Sargassum 
(illus.),  208. 

Pupa,  93. 

Puss  moth,  larva  of  (illus.),  216. 

Quiescence,  degeneration  through, 
193. 

Rabbit,  embryonic  stages  of  (illus.), 
87. 

Radiolaria,  16. 

Radiolaria-ooze,  19. 

Raja  binoculata  (illus.),  140. 

Realm,  Arctic,  297;  Australian, 
303;  Indo-African,  301;  Lemu- 
rian,  302;  Neotropical,  301; 
North  Temperate,  299 ;  of  ani- 
mal life,  297 ;  Patagonian,  302 ; 
South  American,  301. 

Realms,  subordinate,  303. 

Reason,  251. 

Recognition  marks,  22,  129, 223. 

Rectum,  68. 

Reflex  action,  241. 

Remora  (illus.),  173. 

Remora  remora  (illus.),  125. 

Reproduction,  9,  50;  instincts  of, 
249. 

Reproductive  cells,  28,  55. 

Resemblance,  aggressive,  202 ;  gen- 
eral protective,  202;  protective, 
201 ;  special  protective,  207 ;  va- 
riable protective,  204. 


INDEX 


327 


Respiration,  7. 

Resting  spore,  28. 

Reticulura,  67. 

Rhizocrinus  loxotensis  (illus.),  305. 

Rivalry,  adaptations  for,  185. 

Rookeries,  fur-seal  (illus.),  169. 

Rumen,  67. 

Sacculina  (illus.),  187. 

Sacculina,  adult  and  larva  (illus.), 
195. 

Salamander,  embryonic  stages  of 
(illus.),  87. 

Saliva,  67. 

Salmon  leaping  (illus.),  289. 

Salmo  viridens  (illus.),  145. 

Sarcoptes  (illus.),  192. 

Sauba,  ant  mimicked  by  Membra- 
cidce  (illus.),  220. 

Scale,  red  orange  (illus.),  196. 

Schilbeodes  furiosus  (illus.),  132. 

Schistocerca  (illus.),  280. 

Scorpion  (illus.),  127. 

Scorpion-fish  (illus.),  132. 

Sea,  a  barrier  to  distribution,  288. 

Sea,  faunal  areas  of,  304. 

Sea-anemone,  37;  with  algas  in 
body  (illus.),  178. 

Sea-cow  (illus.),  277. 

Sea-cucumber,  alimentary  canal  of 
(illus.),  70. 

Sea-squirt  (illus.),  194. 

Sea-urchin,  metamorphosis  of  (il- 
lus.), 96. 

Sea-urchins  (illus.),  259. 

Seal,  fur  (illus.),  136;  pups  killed 
by  parasite  (illus.),  186;  rook- 
eries (illus.),  169. 

Selection,artificial,120;natural,117. 

Self-defense,  adaptations  for,  128 ; 
instincts  of,  245. 

Sensation,  8. 


Senses,  special,  224 ;  of  the  simplest 
animals,  225. 

Sensorium,  241. 

Serphus  (illus.),  141. 

Sex,  57 ;  object  of,  57 ;  dimorphism, 
58. 

Shark-clinging  fish  (illus.),  125. 

Sheep,  bighorn  (illus.),  167 ;  Rocky 
Mountain  (illus.),  167. 

Sight,  sense  of,  237. 

Simplest  animals,  life  of,  1. 

Siphonophora,  46. 

Skate,  egg  case  of  California  barn- 
door (illus.),  140 ;  egg  of  (illus.), 
79. 

Skin  of  man,  tactile  papilla  of 
(illus.),  227. 

Smell,  sense  of,  229. 

Smelling  organs,  229. 

Smelling  pits  of  leaf-eating  beetle 
(illus.),  230. 

Social  life,  149. 

Sound-making,  235 ;  organs,  235. 

South  American  realm,  301. 

Specialization,  66 ;  of  organs,  66. 

Special  senses,  224. 

Species,  altered  by  adaptation  to 
new  conditions,  276;  debarred 
by  barriers,  274 ;  debarred  by  in- 
ability to  maintain  their  ground, 
275 ;  definition  of,  279 ;  relation 
of,  to  habitat,  283. 

Sperm  cell,  35,  56. 

Sphinx  moth,  larva  of  (illus.),  215. 

Spicules,  sponge,  33. 

Spiders  (illus.),  212;  nest-making 
habits  of,  259. 

Sponges,  32. 

Spongin,  36. 

Spontaneous  generation,  51. 

Spore,  15,  52 ;  resting,  28. 

Sting-ray  (illus.),  133. 


ANIMAL  LIFE 


Structure, 63;  differentiation  of,  64. 

Struggle  for  existence,  116. 

Sty  lops  parasitizing  Polistes  (illus.), 
192. 

Sub-species,  definition  of,  282. 

Surroundings,  adaptations  con- 
cerned with,  143. 

Swallow-tail  butterfly,  chrysalid 
of  (illus.),  205. 

Sword-fish,  metamorphosis  of  (il- 
lus.), 99. 

Symbiosis,  172, 175. 

Syngamus  trachealis  (illus.),  60. 

Systematic  zoology,  64. 

Tactile  organs,  226. 

Tactile  papilla  of  skin  of  man  (il- 
lus.), 227. 

Tadpole,  94. 

Tcenia,  solium  (illus.),  183. 

Tailor-bird,  nest  of  (illus.),  268. 

Tape-worm  (illus.),  183. 

Taste  buds  of  calf  (illus.),  229. 

Taste  organs,  228. 

Taste,  sense  of,  228. 

Temperature  a  barrier  to  distribu- 
tion, 290 ;  a  condition  of  animal 
life,  108. 

Tentacle,  37. 

Termite,  158,  (illus.),  159. 

Terrifying  appearances,  212. 

Thalessa  lunator  (illus.),  191. 

Toad,  egg  of  (illus.),  79;  horned 
(illus.),  131  ;  metamorphosis  of, 
94,  (illus.),  95. 

Torpedo  (illus.),  135. 

Tortoise,  embryonic  stages  of  (il- 
lus.), 87. 

Touch,  sense  of,  226. 

Trap-door  spider  nest  (illus.),  261. 

Tree-hoppers  mimicking  leaf-cut- 
ting ant  (illus.),  220. 


Tree-toad  (illus.),  145. 

Tremex  columba  (illus.),  191. 

Trichechus  latirostris  (illus.),  277. 

Trichina  spiralis  (illus.),  184. 

Tripoli,  19. 

Trochilus  rufus,  nest   of  (illus.), 

265,  266. 
Trout,   rainbow,  head    of  (illus.), 

145. 

Two  Ocean  Pass  (illus.),  287. 
Tunicate  (illus.),  194. 
Turret-spider,  nest  of  (illus.),  262. 
Typhlichthys  subterraneus  (illus.), 

282. 

Uncinaria,  killing    fur-seal  pups 

(illus.),  186. 

Uria  lomvia  arra  (illus.),  165. 
Urolophus  goodei  (illus.),  133. 

Vacuole,  10;  contractile,  10. 

Variety,  definition  of,  282. 

Vedalia  and  Icerya,  121. 

Vertebrates,  early  stages  in  devel- 
opment of  (illus.),  87 ;  nest-mak- 
ing habits  of,  264 ;  parasitic,  193. 

Vespa  (illus.),  162 ;  nest  of  (illus.), 
163. 

Vestigial  organs,  147. 

Viceroy  butterfly  mimicking  Mon- 
arch butterfly  (illus.),  219. 

Voice,  236. 

Volvocinse,  24. 

Volvox,  28. 

Volvox  globator  (illus.),  29. 

Volvox  minor  (illus.),  29. 

Vorticella,  12. 

Vorticella  microtoma  (illus.),  12. 

Walking-stick  insect  (illus.),  209. 
Warning  colors,  212. 
Walrus,  Atlantic  (illus.),  298. 


INDEX 


Wasps,  social,  161. 

Water -beetle    (illus.),    146;    bug, 

giant  (illus.),  141. 
Whippoorwill  (illus.),  203. 
Woodpecker,  Californian,  feeding 

habit  of  (illus.),  128,  129. 

Xiphias   gladius,   metamorphosis 
of  (illus.),  99. 


Yellow-jacket  (illus.),  162. 

Yolk,  80. 

Young,  adaptations  for  defense  of, 
137 ;  care  of  the,  250,  257 ;  num- 
ber of,  61. 

Zoea,  97. 

Zoogeography,  272. 
Zoology,  systematic,  64. 


(5) 


THE   END 


TWENTIETH    CENTURY  TEXT-BOOKS 


ANIMAL   FORMS 

A  TEXT-BOOK  OF  ZOOLOGY 


BY 

DAVID   STARR  JORDAN,   PH.  D. 

PRESIDENT   OF   LELAND    STANFORD    JUNIOR    UNIVERSITY 
AND 

HAROLD  HEATH,  PH.  D. 

ASSISTANT   PROFESSOR   OF    ZOOLOGY,    LELAND    STANFORD   JUNIOR 
UNIVERSITY 


NEW    YORK 

D.    APPLETON    AND    COMPANY 
1907 


COPYRIGHT,  1902 
BY  D.   APPLETON   AND  COMPANY 


Published  May,  1902 


PEEFACE 


THE  present  volume  is  designed  to  meet  the  needs  of 
the  beginning  student  of  zoology.  Accordingly,  technical 
and  scientific  names  have  been  avoided  as  far  as  possible, 
and  those  used  are  fully  explained  in  the  text  or  elsewhere. 
The  opening  chapters  deal  with  the  characteristics  of  living 
things,  and,  in  contrasting  animals  and  plants,  attempt  to 
bring  into  relief  the  distinguishing  marks  of  all  animals. 
Then  follows  a  discussion  of  the  cell  and  protoplasm,  pre- 
paring the  way  for  the  examination  of  a  series  of  animals 
representative  of  each  of  the  great  groups,  from  the  sim- 
plest to  the  most  complex.  These  are  considered  from  the 
view-point  of  structure ;  but  considerable  attention  is  also 
paid  to  the  functions  of  their  parts,  to  their  habits  and  life- 
history,  so  that  while  the  representatives  examined  are,  for 
the  sake  of  simplicity,  relatively  few  in  number,  they  are,  it 
is  believed,  thoroughly  typical.  Hence,  with  a  knowledge 
of  the  facts  presented,  the  student  should  have  a  broad 
view  of  the  animal  kingdom,  and  a  foundation  on  which  to 
base  future  study  and  observation.  It  is  perhaps  unneces- 
sary to  add  that  from  the  study  of  books  alone  no  one  can 
really  make  such  knowledge  his  own.  A  personal  acquaint- 
ance with  even  a  few  animals  in  their  native  haunts,  and 
an  understanding  of  the  structure  and  the  function  of  their 


vi  ANIMAL  FORMS 

parts  gained  from  dissection  and  experiment,  is  essential  to 
a  full  comprehension  of  what  the  student  learns  from  text- 
book and  teacher. 

The  greater  number  of  illustrations  are  new,  and  have 
been  drawn  or  photographed  from  living  or  preserved  ma- 
te"rial.  When  not  otherwise  accredited,  the  drawings  have 
been  made  by  Miss  Mary  H.  Wellman  and  J.  Carter  Beard, 
to  whom  the  authors  extend  their  sincere  thanks.  Our 
obligations  are  also  due  to  Mr.  Walter  K.  Fisher,  who  has 
made  the  drawings  of  the  vertebrate  dissections ;  to  Messrs. 
A.  L.  Melander  and  C.  T.  Brues,  of  Chicago,  111.  ;  Mr.  Wm. 
H.  Fisher,  of  Baltimore,  Md. ;  Eev.  H.  K.  Job,  of  Kent, 
Conn. ;  Mr.  Wm.  Graham,  of  Pasadena,  Cal. ;  and  Dr.  E.  W. 
Shufeldt,  of  New  York  city,  for  numerous  photographs. 

DAYID  STARE  JORDAN, 
HAROLD  HEATH. 


CONTENTS 


CHAPTER  PAGE 

I. — INTRODUCTION 1 

II. — THE   CELL   AND   PROTOPLASM .7 

III.— THE  PROTOZOA 11 

IV. — THE  SPONGES 19 

V. — THE    CCELENTERATES 29 

VI.— THE  WORMS 44 

VII. — ANIMALS  OF  UNCERTAIN  RELATIONSHIPS    ....  66 

VIII.— MOLLUSKS 72 

IX. — ARTHROPODS.    CLASS  CRUSTACEA 93 

X. — ARTHROPODS.    CLASS  INSECTS 114 

XI. — ARTHROPODS.    CLASS  ARACHNIDA 133 

XII.— ECHINODERMS 140 

XIII.— THE  CHORDATES 151 

XIV.— THE  FISHES 154 

XV.— THE  AMPHIBIANS 174 

XVI.— THE  REPTILES 184 

XVII.— THE  BIRDS 201 

XVIII.— THE  MAMMALS 225' 

vii 


ANIMAL   FORMS 


CHAPTER  I 

INTRODUCTION 

1.  Divisions  of  the  subject. — Biology  is  the  science  which 
treats  of  living  things  in  all  their  relations.     It  is  sub- 
divided into  Zoology,  the  science  which  deals  with  animals, 
and  Botany,  which  is  concerned  with  plants.     The  field 
covered    by  each   of  these    branches    is    very  extensive. 
Within  the  scope  of  zoology  are  included  all  subjects  bear- 
ing on  the  form  and  structure  of  animals,  on  their  devel- 
opment, and  on  their  activities,  including  the  consideration 
of  their  habits  and  the  wider  problems  of  their  distribution 
and  their  relations  to  one  another. 

These  various  subjects  are  often  conveniently  grouped 
under  three  heads :  Morphology,  which  treats  of  the  form 
and  structure  or  the  anatomy  of  organisms ;  Physiology, 
which  considers  their  activities;  and  Ecology,  which  in- 
cludes their  relations  one  to  another  and  to  their  surround- 
ings. All  the  phases  of  plant  or  animal  existence  may  be 
considered  under  one  or  another  of  these  three  divisions. 

2.  The  difference  between  animals  and  plants.— Generally 
speaking,  we  have  little  difficulty  in  seeing  that  the  objects 
about  us  are  either  living  or  lifeless  ;  but  the  boundary  line 
between  the  two  great  divisions  of  living  things,  the  animals 
and  plants,  can  not  always  be  so  clearly  drawn.     This  is  es- 
pecially true  of  the  simpler  forms  of  life  which  frequently 
combine  both  animal  and  plant  characteristics ;  but  in  the 

1 


2  ANIMAL  FORMS 

greater  number  of  more  highly  developed  species  the  line 
of  separation  is  clearly  marked.  It  is  very  easy,  for  example, 
to  distinguish  the  oak-tree  or  the  rose  from  a  horse  or  a 
"butterfly,  and,  as  we  shall  see,  the  differences  are  not  based 
merely  on  outward  appearance. 

In  the  oak-tree,  for  example,  the  roots  reaching  down 
into  the  earth,  with  the  branches  and  leaves  spreading  out 
into  the  air  and  sunlight,  are  admirably  fitted  for  taking  up 
the  food,  which  consists  of  very  simple  materials,  less  com- 
plex than  those  forming  the  diet  of  an  animal.  This 
permits  a  continuous  existence  in  one  place,  and  accord- 
ingly we  note  the  entire  absence  of  locomotion  and  the  or- 
gans controlling  it,  which  form  so  conspicuous  a  part  of  the 
body  of  an  animal.  Also  in  the  production  of  flowers  and 
seeds,  and  in  the  growth  of  the  seed  into  the  tree,  we  detect 
many  characteristics  peculiar  to  plants. 

3.  Characteristics  of  an  animal.— On  the  other  hand,  the 
squirrel,  for  example,  or  any  other  animal,  is  unable  to  sub- 
sist on  water,  air,  and  elements  from  the  soil.  These  crea- 
tures demand  the  highly  diversified  materials  found  in  the 
bodies  of  plants  and  of  animals.  Such  being  the  case,  they 
do  not  remain  anchored  to  one  spot  (except  in  a  relatively 
few  cases),  but  are  compelled  to  lead  an  active  existence. 
The  power  of  voluntary  movement,  or  movement  in  response 
to  internal  impulse,  is  thus  the  first  and  one  of  the  most 
striking  peculiarities  of  animals. 

In  the  second  place,  the  food  of  plants  enters  the  body 
in  a  soluble  condition  and  is  readily  transferred  to  the  or- 
gans requiring  it.  While  in  the  animals,  the  nutritive  ma- 
terials pass  into  the  body  in  an  insoluble  state  and  de- 
mand a  varied  preliminary  treatment,  usually  within  a 
special  digestive  tube,  before  they  are  fit  to  be  absorbed. 
In  the  squirrel,  by  way  of  illustration,  the  food  is  first 
ground  to  a  pulp  by  the  action  of  the  teeth,  and,  moistened 
with  saliva,  is  swallowed  and  passed  into  the  stomach, 
where  it  is  subjected  to  the  solvent  action  of  the  gastric 


ANIMAL  FORMS 


juice.     From  the  stomach  it  is  made  to  enter  the  intestine, 
and  is  further  acted  upon  by  fluids  from  the  liver,  the 

pancreas,  and  the  glands  of  the 
intestines  themselves.  Thus 
treated  it  becomes  changed  from 
an  insoluble  state  into  a  fluid 
which  readily  penetrates  the 
coats  of  the  digestive  tract. 

Many  of  the  organs  of  the 
body  are  placed  at  a  considera- 
ble distance  from  the  food  as 
it  comes  through  the  coats  of 
the  stomach  and  intestine.  In 
order  to  supply  them  with  the 
necessary  nourishment  a  distrib- 
uting apparatus  is  required. 
This  is  the  office  performed  by 
the  circulatory  system,  for  as 
rapidly  as  the  food  penetrates 
the  walls  of  the  digestive  tract 
it  enters  the  blood,  and  by  the 
beating  of  the  heart  is  driven 
to  all  parts  of  the  body,  which 
are  thus  continually  kept  in  a 
state  of  repair.  The  blood  serves 
also  to  remove  waste  substances 
from  the  various  structures  or 
organs  of  the  animal  body  and 
to  transfer  them  to  the  kidneys, 
skin,  or  lungs,  which  effect  their 
removal  from  the  body. 

4.  Muscular  and  nervous  sys- 
tems.—Owing  to  the  fact  that 
animals,  as  a  rule,  are  compelled 
to  move  about  in  search  of  food,  we  find  two  highly  devel- 
oped systems,  the  muscular  and  nervous,  which  are  absent 


FIG.  2.— Diagram  of  heart  and  blood- 
vessels of  the  squirrel  or  other 
mammal,  a.o.,  aorta  ;  h,  vessels 
of  head  ;  La.,  left  auricle  ;  Lex., 
vessels  of  lower  extremities  ;  Iff., 
lung ;  l.v.,  left  ventricle  ;  p.a., 
pulmonary  artery  ;  p.v.,  pulmo- 
nary vein  ;  r.a.,  right  auricle  ; 
r.v.,  right  ventricle  ;  #.,  vessels  of 
viscera.  Arteries  are  represented 
by  heavy  walls. 


INTRODUCTION  5 

in  the  plants.  The  first  of  these,  constituting  what  is  usu- 
ally known  as  the  lean  meat,  is  a  relatively  complex  system 
of  organs,  differing  widely  according  to  the  work  performed. 
In  the  higher  animals — the  squirrel,  for  example — there  are 
not  less  than  five  hundred  muscles,  which  are  under  the 
control  of  the  nervous  system. 

The  nervous  system  consists  of  the  brain  and  spinal 
cord,  which  in  the  squirrel  are  concealed  and  protected 


FIG.  3.— Skeleton  of  squirrel,  showing  its  relation  to  the  body. 

within  the  skull  and  back-bone.  From  them  many  nerves 
pass  outward  to  the  muscles,  and  as  many  pass  inward  from 
the  eye,  ear,  nose,  tongue,  or  skin.  By  the  action  of  these 
sense-organs  the  animal  determines  the  nature  of  its  sur- 
roundings, detects  its  food,  recognizes  the  presence  of  its 
enemies,  and  is  thus  able  to  direct  its  movements  to  the 
greatest  advantage. 

5.  Multiplication  of  animals. — The  organs  thus  far  con- 
sidered serve  to  perpetuate  the  animal  as  an  individual ; 
but  some  provision  must  also  be  made  for  the  continuance 
of  the  race.  In  the  economy  of  nature  each  animal  before 


6  ANIMAL  FORMS 

its  death  should  leave  offspring  to  take  the  place  of  the 
parent  when  it  falls  from  the  ranks.  This  is  effected  in 
various  ways.  In  some  of  the  simpler  animals  the  body 
may  divide  into  two  equal  parts,  each  of  which  becomes  a 
complete  individual.  In  other  cases  the  animal  detaches 
a  relatively  small  portion  of  its  body,  much  as  a  gardener 
cuts  a  slip  from  a  plant,  and  this  likewise  develops  into  a 
new  organism.  In  the  greater  number  of  animals,  very 
clearly  illustrated  by  the  birds,  eggs  are  produced  which 
under  favorable  conditions  develop  into  an  organism  resem- 
bling the  parents. 

6.  Summary. — Animals  are  thus  seen  to  lead  active,  busy 
lives,  collecting  food,  avoiding  enemies,  and  producing  and 
and  caring  for  their  young.  While  the  activities  of  all 
animals  are  directed  to  their  own  preservation  and  to  the 
multiplication  of  their  kind,  these  processes  are  carried  on 
in  the  most  diverse  ways.  The  manner  in  which  an  organ 
or  an  organism  is  made,  and  the  method  by  which  it  does 
its  work,  are  mutually  dependent  one  on  the  other.  As 
there  is  an  enormous  number  of  species  of  animals,  each 
differently  constructed,  there  is,  accordingly,  a  very  great 
variety  of  habits.  As  we  shall  see,  the  lower  forms  are 
remarkably  simple  in  their  construction,  and  their  mode  of 
existence  is  correspondingly  simple.  In  the  higher  types 
a  much  greater  complexity  exists,  and  their  activities  are 
more  varied  and  are  characterized  by  a  high  degree  of  elabo- 
ration. In  every  case,  the  animal,  whether  high  or  low,  is 
fitted  for  some  particular  haunt,  where  it  may  perform  its 
work  in  its  own  special  way  and  may  lead  a  successful  life 
of  its  own  characteristic  type. 


CHAPTER  II 

THE   CELL  AND   PROTOPLASM 

7.  Cells. — If  we  examine  very  carefully  the  different  parts 
of  a  squirrel  under  the  high  powers  of  the  microscope 
we  find  that  they  are  composed  of  a  multitude  of  small 
structures  which  bear  the  same  relations  to  the  various 
organs  that  bricks  or  stones  do  to  a  wall ;  and  if  the  inves- 
tigation were  continued  it  would  be  found  that  every  or- 
ganism is  composed  of  one  or  more  of  these  lesser  elements 
which  bear  the  name  of  cells.    In  size  they  vary  exceedingly, 
and  their  shapes  are  most  diverse,  but,  despite  these  differ- 
ences, it  will  be  seen  that  all  exhibit  a  certain  general  re- 
semblance one  to  the  other. 

8.  Shape  of  cells. — In  many  of  the   simpler  organisms 
the  component  cells  are  jelly-like  masses  of  a  more  or  less 
spherical  form,  but  as  we  ascend  the  scale  of  life  the  condi- 
tion of  affairs  becomes  much  more  complex.     In  the  squir- 
rel, for  example,  we  have  already  noted  the  presence  of 
various  organs  for  carrying  on  different  functions,  such  as 
those  of  digestion,  circulation,  and  respiration ;  and,  further, 
the  cells  composing  these  various  parts  have  been  modified 
in  accordance  with  the  duties  they  have  to  perform.     In 
the  muscles  the  cells  are  long  and  slender  (Fig.  4,  D) ; 
those  forming  the  nerves  and  conveying  sensations  to  and 
from  all  parts  of  the  body,  like  an  extensive  telegraph  sys- 
tem, are  excessively  delicate  and  thread-like ;  in  the  skin, 
and  lining  many  cavities  of  the  body,  where  the  cells  are 
united  into  extensive  sheets,  they  range  in  shape  from  high 
and  columnar  to  flat  and  scale-like  forms  (Fig.  4,  E,  F,  G). 

7 


8  ANIMAL  FORMS 

The  cells  of  the  blood  present  another  type  (Fig.  4,  B) ;  and 
so  we  might  pass  in  review  other  parts  of  the  body,  and  con- 
tinue our  studies  with  other  groups  of  animals,  always  find- 
ing new  forms  dependent  upon  the  part  they  play  in  the 
organism. 

9.  Size  of  cells. — Also  in  the  matter  of  size  the  greatest 
variations  exist.     Some  of  the  smallest  cells  measure  less 
than  one  micromillimeter  (^TO-^  of  an  inch)  in  diameter. 
Over  five  hundred  million  such  bodies  could  be  readily 
stowed  away  into  a  hollow  sphere  the  size  of  the  letter  be- 
ginning this  sentence.     In  a  drop  of  human  blood  of  the 
same  size,  between  four  and  five  million  blood-cells  or  cor- 
puscles float.     And  from  this  extreme  all  sizes  exist  up  to 
those  with  a  diameter  of  2.5  or  5  c.m.  (one  or  two  inches), 
as  in  the  case  of  the  hen's  or  ostrich's  egg.     On  the  average 
a  cell  will  measure  between  .025  to  .031  m.m.  (TTiVfr  an(^ 
-g-J-o  of  an  inch)  in  diameter,  a  speck  probably  invisible  to 
the  unaided  eye.     While  the  size  and  external  appearance 
of  a  cell  are  seen  to  be  most  variable,  the  internal  structures 
are  found  to  show  a  striking  resemblance  throughout.     All 
are  constructed  upon  essentially  the  same  plan.     Differ- 
ences in  form  and  size  are  superficial,  and  in  passing  to  a 
more  careful  study  of  one  cell  we  gain  a  knowledge  of  the 
important  features  of  all. 

10.  A  typical  cell. — Some  cell,  for  example  that  of  the 
liver  (Fig.  4,  A),  may  be  chosen  as  a  good  representative  of 
a  typical  cell.     To  the  naked  eye  it  is  barely  visible  as  a 
minute  speck ;  but  under  the  microscope  the  appearance  is 
that  of  so  much  white  of  egg,  an  almost  transparent  jelly- 
like  mass  bearing  upon  its  outer  surface  a  thin  structure- 
less membrane  that  serves  to  preserve  its  general  shape  and 
also  to  protect  the  delicate  cell  material  within.    The  com- 
parison of  the  latter  substance  to  egg  albumen  can  be  car- 
ried no  further  than  the  simple  physical  appearance,  for 
albumen  belongs  to  that  great  class  of  substances  which 
are  said  to  be  non-living  or  dead,  while  the  cell  material 


THE  CELL  AND  PROTOPLASM 


9 


or  protoplasm,  as  it  is  termed,  is  a  living  substance.  We 
know  of  no  case  where  life  exists  apart  from  protoplasm, 
and  for  this  reason  the  latter  is  frequently  termed  the 
physical  basis  of  life. 

In  addition  to  the  features  already  described,  the  proto- 
plasm of  every  perfect  cell  is  modified  upon  the  interior  to 


FIG.  4.— Different  types  of  cells  composing  the  body  of  the  squirrel  or  other  highly 
developed  animal.  A,  liver-cell;  /,  food  materials;  n,  nucleus.  B,  blood-cell. 
C,  nerve-cell  with  small  part  of  its  fiber.  D,  muscle  fiber.  E,  cells  lining  the 
body  cavity.  F,  lining  of  the  windpipe.  G,  section  through  the  skin.  Highly 
magnified. 

form  a  well-defined  spherical  mass  known  as  the  nucleus. 
Other  structures  are  known  to  occur  in  the  typical  cell. 
Experiment  shows  that  the  nucleus  and  cell  protoplasm  are 
absolutely  indispensable,  whatever  their  size  and  shape,  and 


10  ANIMAL  FORMS 

therefore  we  are  at  present  justified  in  defining  the  cell  as 
a  small  mass  of  protoplasm  enclosing  a  nucleus. 

11.  Structure  of  protoplasm. — When  seen  under  a  lens 
of  moderate  power  protoplasm  gives  no  indication  of  any 
definite  structure,  and  even  with  the  highest  magnifica- 
tion it  presents  appearances  which  are  not  clearly  under- 
stood. According  to  the  commonly  accepted  view,  it  con- 
sists of  two  portions,  one,  the  firmer,  forming  an  excessively 
delicate  mesh  work  (Fig.  4,  A)  enclosing  in  its  cavities 
the  second  more  fluid  part.  Therefore,  when  highly  mag- 
nified, the  appearance  would  be  essentially  like  a  sponge 
fully  saturated  with  water ;  but  it  should  be  remembered 
that  in  the  protoplasm  the  sponge  work,  and  possibly  the 
fluid  part,  is  living,  and  that  both  are  transparent. 

There  are  reasons  for  thinking  that  the  structure  and 
the  composition  of  protoplasm  may  change  somewhat  under 
certain  circumstances.  It  certainly  is  not  everywhere  alike, 
for  that  of  one  animal  must  differ  from  that  of  another,  and 
different  parts,  such  as  the  liver  and  brain,  of  the  same  form 
must  be  unlike.  These  differences,  however,  are  minor 
when  compared  to  the  resemblances,  for,  as  we  shall  see, 
this  living  substance,  wherever  it  exists,  carries  on  the  pro- 
cesses of  waste,  repair,  growth,  sensation,  contraction,  and 
the  reproduction  of  its  kind. 


CHAPTEE  III 

THE    PROTOZOA 

12.  Single-celled   and    many-celled   animals.— In  almost 
every  portion  of  the  globe  there  are  multitudes  of  animals 
whose  body  consists  of  but  a  single  cell ;  while  those  forms 
more  familiar  to  us,  and  usually  of  comparatively  large 
size   and  higher  development,   such   as   sponges,  insects, 
fishes,  birds,  and  man  himself,  are  composed  of  a  multitude 
of  cells.     For  this  reason  the  animal  kingdom  has  been 
divided  into  two  great  subdivisions,  the  Protozoa  including 
all  unicellular  forms  and  the  Metazoa  embracing  those  of 
many  cells. 

13.  Single-celled  animals. — The  division  of  the  Protozoa 
comprises  a  host  of  animals,  usually  of  microscopic  size, 
inhabiting  fresh  or  salt  water  or  damp  localities  on  land  in 
nearly  every  portion  of  the  globe.     The  greater  number 
wage  their  little,  though  fierce,  wars  on  one  another  with- 
out attracting  much  attention ;  others,  in  the  sharp  struggle, 
have  been  compelled  to  live  upon  or  within  the  bodies  of 
other  animals,  and  many  have  become  notorious  because  of 
the  diseases  they  produce  under  such  circumstances.     A 
few  are  in  large  measure  responsible  for  the  phosphores- 
cence of  the  sea ;  and  still  others  have  long  been  favorite 
objects  of  study  because  of  their  marvelous  beauty.   Adapted 
for  living  under  diverse  conditions,  the  bodily  form  differs 
greatly,  and  yet  all  conform  to  three  or  four  principal  types, 
of  which  we  may  gain  a  good  idea  from  the  study  of  a  few 
representative  forms. 

11 


12 


ANIMAL  FORMS 


14.  The  Amoeba. — Among  the  simplest  one-celled  ani- 
mals living  in  the  ooze  at  the  bottom  of  nearly  every  fresh- 
water stream  or  pond  is  the  Amceba  (Fig.  5,  A),  whose  body 
is  barely  visible  to  the  unaided  eye.  Under  the  microscope 


Fio.  5. — A,  the  Amoeba,  highly  magnified,  showing  c.  v.,  pulsating  vacuole  ;  f,  food 
particle  ;  n,  nucleus.  The  arrows  show  the  direction  of  movement.  B,  shape  of 
same  individual  30  seconds  later.  C,  an  amoeba-like  animal  (Difflugia)  partially 
enclosed  in  a  shell.  D,  an  Amoeba  in  the  process  of  division.  E,  Gromia,  another 
shelled  protozoan  (after  SCHULZE). 

it  is  seen  to  consist  of  an  irregular,  jelly-like  mass  of  proto- 
plasm totally  destitute  of  a  cell  wall.  Unlike  those  animals 
with  which  we  are  familiar,  the  body  constantly  changes  its 
shape.  A  rounded  bud-like  projection  will  be  seen  to  appear 
on  one  side  of  the  body  and  the  protoplasm  of  adjacent 
regions  flows  into  it,  thereby  increasing  its  extent.  Similar 
projections  at  the  opposite  end  of  the  cell  are  withdrawn, 
and  their  substance  may  flow  into  the  newly  formed  lobe, 
which  gradually  swells  in  size  and  pushes  forward.  Thus, 
by  constantly  advancing  the  front  part  of  the  body  and 


THE  PROTOZOA  13 

retracting  the  hinder  portion,  the  cell  glides  or  flows  along 
from  place  to  place. 

Upon  meeting  with  any  of  the  smaller  organisms  upon 
which  it  lives,  projections  from  the  body  are  put  out  which 
gradually  flow  around  the  prey  and  it  becomes  pressed  into 
the  interior  of  the  cell.  The  process  is  not  unlike  pushing 
a  grain  of  sand  into  a  bit  of  jelly.  There  is  no  mouth. 
Any  point  on  the  surface  serves  for  the  reception  of  food. 
Oxygen  gas  also  is  taken  into  the  body  all  over  the  surface, 
and  wastes  and  indigestible  material  are  cast  out  at  any 
point.  Nothing  exists  in  these  simple  forms  comparable  to 
the  complex  systems  of  organs  that  carry  on  these  processes 
in  the  squirrel. 

The  bodily  size  of  animals  is  limited,  and  to  this  general 
rule  the  Amoeba  is  no  exception,  for  upon  gaining  a  certain 
size,  the  nucleus  divides  into  two  exactly  similar  portions, 
and  very  soon  afterward  the  rest  of  the  body  separates  into 
two  independent  masses  of  equal  size  (Fig.  5,  D),  each  of 
which,  when  entirely  free,  contains  a  nucleus.  In  this  way 
two  daughter  amoebae  are  formed  possessing  exactly  the 
characters  of  the  parent  save  that  they  are  of  smaller  size ; 
but  it  is  usually  not  long  before  they  reach  their  limit  of 
growth,  when  division  occurs  again,  and  so  on,  generation 
after  generation. 

It  not  infrequently  happens,  however,  that  the  pond  or 
stream,  in  which  the  Amoeba  and  other  Protozoa  live,  dries 
up  for  a  portion  of  the  year.  In  such  an  event  the  body 
assumes  a  spherical  shape,  develops  a  firm,  horn-like  mem- 
brane about  itself,  and  thus  encysted  it  withstands  the  sum- 
mer's heat  and  dryness  and  may  be  transported  by  the  wind, 
or  otherwise,  over  great  distances.  When  the  conditions 
again  become  favorable  the  wall  ruptures  and  the  Amoeba 
emerges  to  repeat  its  life  processes. 

15.  Some  relatives  of  the  Amoeba. — All  amceba-like  forms, 
to  the  number  of  perhaps  a  thousand  species,  possess  this 
same  method  of  locomotion,  but  many  present  some  inter- 


14  ANIMAL  FORMS 

esting  additional  characters.  For  example,  the  form  repre- 
sented in  Fig.  5,  C,  constructs  a  sac-like  skeleton  of  tiny 
pebbles  cemented  together,  into  which  it  may  withdraw  for 
protection.  Others  construct  similar  envelopes  of  lime  or 
flint,  and  still  others,  as  they  continue  to  grow,  build  on 
additional  chambers,  giving  rise  to  a  great  variety  of  forms 
often  of  wonderful  beauty.  In  the  tropics,  particularly, 
some  of  the  shelled  Protozoa  are  so  abundant  that  they  may 
impart  a  whitish  tinge  to  the  water,  and  in  some  places 
their  empty  shells  on  falling  to  the  bottom  form  immense 
deposits.  The  chalk  cliffs  of  England  are  in  large  measure 
made  up  of  such  shells. 

16.  The  Infusoria. — A  little  over  two  hundred  years  ago 
it  was  discovered  that  wherever  water  remained  stagnant  it 
became  favorable  for  the  rapid  multiplication  of  a  large 
number  of  species  of  Protozoa  which  live  in  such  situations. 
These  are  known  as  Infusoria,  and,  like  the  preceding  spe- 
cies, are  usually  of  microscopic  size  and  of  the  most  varied 
shapes.     The  first  striking  feature  of  their  organization  is 
the  presence  of  a  delicate  though  relatively  firm  external 
cell  membrane  known  as  the  cuticle,  which  preserves  a  defi- 
nite shape  to  the  body.     Such  a  method  of  locomotion  as 
exists  in  the  preceding  group  is  consequently  an  impossi- 
bility, but  other  and  more  highly  developed  structures  per- 
form the  office.     These  latter  organs  are  of  two  types,  and 
their  general   characteristics  may  be  readily  understood 
from  an  examination  of  a  few  species  living  in  the  same 
localities  as  the  Amceba. 

17.  The  Euglena. — The  first  type  exists  in  the  common 
fresh-water  organism   known  as   Euglena,  represented  in 
Fig.  6,  A.     Here  the  spindle-shaped  body  is  surrounded  by 
a  delicate  cuticle  perforated  at  one  point,  where  a  funnel- 
shaped  depression,  the  gullet,  leads  into  the   soft  proto- 
plasmic interior.     From  the  base  of  this   depression  the 
protoplasm  is  drawn  out  in  the  form  of  a  delicate  whip-like 
process  known  as  the  flagellum.     This  structure,  always 


THE  PROTOZOA 


15 


permanent  in  form,  constantly  beats  backward  and  forward 
with  great  rapidity  in  a  general  direction  represented  in 
the  diagram  (Fig.  6,  c).  The  movement  from  a  to  b  is 
much  more  rapid  than  the  reverse,  from  b  to  «,  which 
results,  like  the  action  of  the  human  arm  in  swimming,  in 
driving  the  organism  forward.  Not  only  does  the  flagel- 
lum  serve  the  purpose  of  locomotion,  but  it  also  produces 
currents  in  the  water  which 
may  serve  to  bear  minute 
organisms  down  into  the 
"gullet,  whence  they  read- 
ily pass  into  the  soft  pro- 


FIG.  6.  —  Flagellate  Infusoria.  A, 
Euglena  viridis  ;  c,  pulsating 
vacuole  ;  e,  eye-spot ;  g,  gullet ; 
w,  nucleus  ;  t,  flagellum.  B,  Co- 
dosiga,  with  collar  surrounding 
the  flagellum.  C,  diagram  illus- 
trating the  action  of  the  flagel- 
lum. All  figures  greatly  enlarged. 


FIG.  7.  —  Paramcecium  aurelia,  a 
ciliate  infusorian.  c,  cilia;  c.v., 
pulsating  vacuoles  ;  /,  food 
particles  ;  g,  gullet ;  m,  buccal 
groove ;  n,  nucleus. 


toplasm  of  the  body,  there  to  undergo  the  processes  of  di- 
gestion and  assimilation.  In  some  forms  the  protoplasm  in 
the  region  of  the  flagellum  is  drawn  out  in  the  form  of  a 
collar  (Fig.  6,  B),  whose  vibratory  motion  also  aids  in  con- 
veying and  guiding  food  into  the  body. 

18.  The  Slipper  Animalcule. — The  second  type  of  loco- 
motor  organ    may  be  understood  from  a  study  of   the 
24 


16 


ANIMAL  FORMS 


Slipper  Animalcule  (Paramwcium,  Fig.  7),  abundant  in 
stagnant  water.  In  this  form  the  cuticle  surrounding  the 
somewhat  cylindrical  body  is  perforated  by  a  great  number 
of  minute  openings  through  which  the 
internal  protoplasm  projects  in  the  form 
of  delicate  threads.  Each  process, 
termed  a  cilium,  works  on  the  same 
principle  as  the  flagellum,  but  it  beats 
with  an  almost  perfect  rhythm  and  in 
unison  with  its  fellows,  drives  the  an- 
imal hither  and  thither  with  considera- 
ble rapidity. 

On  one  side  of  the  body  is  a  furrow 
which  deepens  as  it  runs  backward  and 
finally  passes  into  the  gullet  (#),  which 
leads  into  the  interior  of  the  body. 
Throughout  the  entire  extent  it  is  lined 
with  cilia  which  create  strong  currents 
in  the  surrounding  water  and  in  this 
way  conduct  food  down  the  gullet  into 
the  body.  Embedded  in  the  outer  sur- 
face of  the  body,  in  among  the  cilia, 
are  also  a  number  of  very  minute  sacks, 
each  containing  a  coiled  thread  which 
may  be  discharged  against  the  body  of 
any  intruder,  so  that  this  form  is  sup- 
plied with  actual  organs  of  defense. 
Two  pulsating  vacuoles  (c.v.)  or  simple 
kidneys  are  also  present,  consisting  of  a 
central  reservoir  into  which  a  number 
of  radiating  canals  extend. 

19.  The  Bell  Animalcule  and  other 
species,— The  Bell  Animalcule  ( Vorti- 
cella,  Fig.  8)  is  often  found  in  the  same  situations  as  the 
Slipper  Animalcule,  which  in  certain  respects  it  resembles. 
It  is  generally  attached  by  a  slender  stalk,  and  where  many 


FIG.  8.—Vorticella,  an  at- 
tached ciliate  infusori- 
an,  highly  magnified,  a, 
fully  extended  individ- 
ual ;  c.v.,  pulsating  va- 
cuole  ;  g,  gullet ;  n,  nu- 
cleus, b,  contracted 
specimen,  c,  small  free- 
swimming  individual, 
which  unites  with  a  sta- 
tionary individual  (one 
partly  united  is  shown 
in  specimen  b). 


THE  PROTOZOA  1Y 

are  growing  together  they  appear  like  a  delicate  growth 
of  mold  upon  the  water  weed.  The  stalk  is  peculiar  in 
being  traversed  by  a  muscle  fiber  arranged  in  a  loose  spiral, 
which  upon  any  unusual  disturbance  contracts  together 
with  the  body  into  the  form  shown  in  Fig.  8,  b. 

These  few  examples  serve  to  show  the  general  plan  of 
organization  and  the  method  of  locomotion  of  the  Infuso- 
ria ;  but,  as  upward  of  a  thousand  species  exist,  with  widely 
differing  habits,  many  interesting  modifications  are  present. 
Some  have  been  driven  in  past  time  to  adopt  a  parasitic 
mode  of  life  within  the  bodies  of  other  animals.  At  pres- 
ent they  are  devoid  of  locomotor  organs,  and  as  they  absorb 
nutritive  fluids  through  the  surface  of  the  body  all  traces 
of  a  mouth  are  also  absent.  The  reproductive  processes 
also  are  peculiar,  but  they  do  not  concern  us  now. 

20.  Characteristics  common  to  the  Protozoa. — We  have 
now  studied  the  principal  structures  which  serve  in  loco- 
motion among  these  simple  one-celled  forms,  also  the  means 
by  which  they  catch  their  food,  and  we  shall  now  glance  at 
the  internal  processes,  which  are  much  the  same  in  all. 

After  the  food  has  been  taken  into  the  cell,  it  is  prob- 
ably acted  upon  by  some  digestive  fluid,  for  it  soon  assumes 
a  granular  appearance  and  finally  undergoes  complete  solu- 
tion. In  every  case  the  oxygen  is  absorbed  through  the 
general  surface  of  the  body,  and  uniting  with  the  living 
substance,  as  in  the  squirrel,  liberates  the  energy  necessary 
for  the  performance  of  the  animal's  life  work.  The  wastes 
thus  produced  in  a  large  number  of  forms  simply  filter  out 
from  the  body  without  the  agency  of  anything  comparable 
to  a  kidney,  but  in  several  species  they  are  borne  to  a 
definite  spot,  the  pulsating  vacuole  (Figs.  5, 7,  8,  c.v.),  where 
they  gradually  accumulate  into  a  drop  about  the  size  of  the 
nucleus.  The  wall  between  it  and  the  exterior  now  gives 
way  and  the  excretions  are  passed  out.  In  active  indi- 
viduals this  process  may  be  repeated  two  or  three  times  a 
minute,  but  it  is  usually  of  less  frequent  occurrence. 


18  ANIMAL  FORMS 

The  loss  in  bodily  waste  is  continually  made  good  by 
the  manufacture  of  the  food  into  protoplasm,  and  if  the  in- 
come be  greater  than  the  outgo  growth  ensues.  But,  as  in 
all  other  forms,  growth  is  limited,  and  ultimately  the  cell  is 
destined  to  divide,  resulting  in  two  new  individuals.  This 
process  may  be  repeated  many  times,  but  not  indefinitely, 
for  sooner  or  later  various  members  of  the  same  species 
unite  in  pairs  temporarily  or  permanently,  exchange  nu- 
clear material,  and  separate  again  with  apparently  renewed 
energy  and  the  ability  to  divide  for  many  generations. 

21.  Simple  and  complex  animals. — It  is  important  to  note 
that  these  same  processes  of  waste,  repair,  growth,  feeling, 
motion,  and  multiplication  are  the  same  as  those  of  the 
squirrel,  and,  furthermore,  are  common  to  all  living  crea- 
tures, so  that  the  difference  between  animals  is  not  in  their 
activities,  but  in  their  bodily  mechanisms  ;  and  according  to 
the  perfection  of  this,  the  animal  is  high  or  low  in  the 
scale.  Comparing,  for  example,  the  Amceba  and  Slipper 
Animalcule,  which  are  relatively  low  and  high  Protozoa,  we 
find  in  the  former  that  any  part  of  the  body  serves  in  loco- 
motion and  in  the  capture  of  food,  while  in  the  latter  these 
same  functions  are  performed  by  definite  structures,  the 
and  gullet.  Now  it  is  well  known  that  a  workman  is 
able  to  make  better  watch-springs,  when  this  is  his  sole 
duty,  than  another  who  must  make  all  parts  of  the  watch ; 
and  likewise  where  a  definite  task  is  performed  by  a  defi- 
nite structure,  it  is  more  efficiently  done  than  where  any 
and  every  part  of  the  body  must  carry  it  on.  So  the 
Amceba,  in  which  definite  tasks  are  performed  by  any  part 
of  the  body  indifferently,  is  less  perfect  and  thus  lower  than 
the  Paramcecium,  where  these  functions  are  performed  by 
special  organs.  As  we  ascend  the  scale  of  life  we  find  this 
division  of  labor  among  special  parts  of  the  body  more 
complete,  the  organs  and  therefore  the  animal  more  com- 
plex, and  better  fitted  to  carry  on  the  work  of  its  life. 


CHAPTER  IV 


THE   SPONGES 

22.  Their  relation  to  the  Protozoa. — While  the  greater 
number  of  one-celled  forms  are  not  united  with  their  fel- 
lows, there  are  several  species  where  the  reverse  is  true.  In 
Fig.  9,  for  example,  a  fresh-water  form  known  as  Pandorina 
is  represented,  consisting  of  sixteen  cells  embedded  in  a 
spherical,  jelly-like  substance, 
each  one  of  which  is  precisely 
like  its  companions  in  form 
and  activity.  The  aggregation 
may  be  looked  upon  as  a  colo- 
ny of  sixteen  Protozoa  united 
together  to  derive  the  benefit 
of  increased  locomotion  and 
a  larger  amount  of  food  in 
consequence.  As  a  result  of 
such  a  union  they  have  not 
lost  their  independence,  for  if 
one  be  separated  from  the  main 
company  it  continues  to  exist. 

From  such  a  simple  colonial 

type  we  may  pass  through  a  series  of  several  more  complex 
forms  which  reach  their  highest  development  in  the  beau- 
tiful organism,  Volvox  (Fig.  10).  In  this  form  the  indi- 
vidual members,  to  the  number  of  many  thousand,  are  ar- 
ranged in  the  shape  of  a  hollow  sphere.  The  united  efforts 
of  the  greater  number,  which  bear  on  their  outer  surfaces 
two  flagella,  drive  the  colony  with  the  rolling  movement 

19 


FIG.  9.— Pandorina  (from  Nature). 
Highly  magnified. 


ANIMAL  FORMS 


from  place  to  place.  As  just  indicated,  some  individuals 
lack  the  flagella,  and  their  subsequent  careers  show  them 
to  be  of  a  peculiar  type.  Sooner  or  later  each  undergoes 

a  series  of  divisions  form- 
ing a  little  globe  of  cells, 
which  migrates  into  the  in- 
terior of  the  parent  sphere 
and  develops  into  a  new 
colony.  Within  a  short  time 
the  walls  of  the  parent 
break,  liberating  the  im- 
prisoned young,  which  con- 
tinue the  existence  of  the 
species  while  the  parent  or- 
ganism soon  decays. 

Under  certain  circum- 
stances, instead  of  develop- 
ing colonies  by  such  a  meth- 
od, some  of  the  cells  may 
store  up  food  matters  and 
become  eggs,  while  others, 
known  as  sperm-cells,  de- 
velop a  flagellum,  and  sep- 
arating from  the  colony 
swim  actively  in  the  sur- 
rounding water,  where  each 
finally  unites  with  an  egg. 
This  union,  like  that  of  the 
two  individuals  in  Vorticel- 
la  (Fig.  8,  #,  c),  results  in 

PIG.  10.-A,  Volvox  minor,  entire  colony  tne  P<>Wer  of  division,  and 
(from  Nature).  B,  C,  and  D,  reproduc-  the  egg  enters  upon  its  de- 
tive  cells  of  Volvox  globator.  All  highly  ,  -, .  .  j . 

magnified.  velopment,   dividing  again 

and  again.  The  cells  so  pro- 
duced remain  together,  form  a  sphere,  and  finally  develop 
a  Volvox  colony. 


THE  SPONGES  21 

In  such  associations  as  Volvox  an  important  step  has 
been  taken  beyond  that  of  Pandorina,  for  there  is  a  division 
of  the  labors  of  the  colony  among  its  various  members, 
some  acting  as  locomotor  cells  while  others  are  germ-cells. 
These  are  now  so  dependent  one  upon  the  other  that  they 
are  unable  to  exist  after  separation  from  the  main  com- 
pany, just  as  a  part  of  the  squirrel  is  incapable  of  leading 
an  independent  existence.  A  higher  type  of  organism  has 
thus  arisen  intermediate  between  the  simple  one-celled 
animals  and  those  of  many  cells,  especially  the  sponges — a 
relation  which  is  more  readily  recognized  after  an  examina- 
tion of  the  latter. 

23.  Development  of  the  sponge.— Like  all  many-celled 
animals,  the  sponge  begins  its  life,  however,  as  a  single  cell 
— the  egg — which  is  in  this  case  barely  visible  to  the  sharp 
unaided  eye.  Fertilized  by  its  union  with  a  sperm  cell, 
development  commences,  and  the  first  apparent  indication 
of  the  process  will  be  the  division  of  the  cell  into  halves 
(Fig.  11,  A,  B).  Each  half  redivides  into  four,  these  again 
into  eight  cells,  and  this  process  is  repeated,  giving  the 
young  sponge  the  general  form  of  Pandorina.  The  divi- 
sions of  the  cells  still  continue  and  result  in  the  formation 
of  a  hollow  globe  of  cells  (called  the  Uastula,  Fig.  11,  E,  F) 
similar  to  Volvox,  and  at  this  point  the  young  larva  leaves 
the  parent. 

The  next  transformation  consists  in  a  pushing  in  of  one 
side  of  the  sphere,  just  as  one  might  press  in  the  side  of  a 
hollow  rubber  ball.  The  depression  gradually  deepens,  and 
finally  results  in  the  formation  of  a  two-layered  sac  known 
as  the  gastrula  (Fig.  11,  G).  At  this  stage  of  its  existence 
the  sponge  settles  down  for  life  in  some  suitable  spot,  by 
applying  the  opening  of  its  sac-like  body  to  some  foreign 
object.  In  assuming  the  final  form  a  new  mouth  breaks 
through  what  was  once  the  bottom  of  the  sac,  canals  per- 
forate the  body  wall,  a  skeleton  is  developed,  and  the  char- 
acteristic  features  of  the  adult  are  thus  attained. 


22 


ANIMAL  FORMS 


24.  Distribution. — The  sponges  are  aquatic  animals,  and, 
with  the  exception  of  one  family  consisting  of  relatively 
few  species,  all  are  inhabitants  of  the  sea  in  every  part  of 


FIG.  11. — Diagrams  illustrating  the  development  of  a  sponge.  A,  egg-cell ;  n,  nucleus. 
B,  C,  D,  2-,  4-,  and  16-cell  stages.  E,  Uastula,  F,  section  through  somewhat 
older  larva.  G,  gastrula.  H,  young  sponge.  I,  section  through  somewhat 
younger  larva  than  H. 

the  glohe.  The  larger  number  occupy  positions  along  the 
shore,  becoming  especially  abundant  in  the  tropics ;  but 
other  species  occur  at  greater  depths,  several  species  living 


THE  SPONGES  23 

between  three  and  four  miles  from  the  surface.  Unlike 
the  majority  of  animals,  all  members  of  this  group  are 
securely  fastened  to  some  foreign  object,  such  as  rocks,  the 
supports  of  wharves,  or  with  one  extremity  embedded  in 
the  sand.  As  we  have  seen,  the  young  enjoy  a  free-swim- 
ming existence  and  are  swept  far  and  wide  by  means  of 
tidal  currents,  but  sooner  or  later  these  migrations  are 
terminated  in  some  suitable  locality,  where  the  sponge 
passes  the  remainder  of  its  existence.  During  this  time 
some  species  may  never  exceed  the  size  of  a  mustard-seed, 
while  others  attain  a  diameter  of  three  feet,  or  even  more. 
Sponges  also  vary  exceedingly  in  shape,  some  having  the 
form  of  thin  encrusting  sheets,  others  being  globular,  tubu- 
lar, cuplike,  or  highly  branched  (Fig.  12). 

25.  The  influence  of  their  surroundings. — In  by  far  the 
larger  number  of  cases  an  animal  possesses  the  bodily  form 
of  the  parent.  External  agencies  may  modify  this  to  some 
extent,  but  usually  only  to  a  limited  degree.  A  squirrel, 
for  example,  resembling  its  parent,  may  grow  to  a  relatively 
large  or  stunted  size  according  to  the  food  supply,  and  it 
may  become  strong  or  weak  according  to  the  amount  of 
exercise,  and  various  other  changes  may  result  owing  to 
outside  causes ;  but  as  a  result  of  these  influences  the 
animal  is  rarely  so  modified  that  one  is  unable  to  distinguish 
the  species.  Many  of  the  sponges,  however,  are  exceptions 
to  this  general  rule.  If,  for  example,  some  of  the  young 
of  a  certain  parent  develop  in  quiet  water  or  in  an  un- 
favorable locality,  they  will  usually  be  low,  flat,  and  un- 
branched ;  while  the  others,  growing  in  swiftly  running 
waterways,  develop  into  tall,  comparatively  delicate  and 
highly  branched  individuals.  Under  such  circumstances 
not  only  does  the  external  form  become  modified,  but 
the  internal  organization  may  undergo  profound  change. 
The  entire  organism  is  plastic  and  readily  molded  by 
the  influence  of  its  surroundings,  and  the  consequent 
lack  of  definite  characters  often  renders  it  impossible 


ANIMAL  FORMS 


to  assign   such  forms  to  a  definite  position  among  the 

sponges. 

26.  Structure  of  a  simple  sponge.— In  the  simpler  sponges 

the  body  is  usually  vase-shaped  (Fig.  13),  with  the  base 

fastened  to  some  foreign 
object,  while  at  an  oppo- 
site end  an  opening  leads 
into  a  comparatively  large 
internal  cavity.  This  lat- 
ter space  is  also  put  in 
communication  with  the 
exterior  by  a  multitude  of 
minute  pores  which  pene: 
trate  the  body  wall.  In 


FIG.  12.— Various  forms  of  sponges,  natural  size.    (From  Nature.) 

the  living  condition  currents  of  water  continually  pass 
through  these  smaller  canals,  and  out  of  the  large  termi- 
nal opening,  thus  bringing  within  reach  of  the  body  minute 


THE  SPONGES 


floating  organisms  or  organic  remains  which  serve  as  food. 
The  mechanism  by  which  this  process  is  effected,  and  the 
various  other  structures  of  the  body,  are  in  large  part  invis- 
ible from  the  exterior,  requiring  the 
study  of  thin  sections  of  the  sponge 
to  make  them  clearly  understood. 

Under  the  microscope  such  a  sec- 
tion shows  the  body  of  a  sponge  to 
consist  of  an  immense  number  of  va- 
riously formed  cells  constituting  three 
distinct  layers  (Fig.  14).  Not  only 
do  these  layers  consist  of  different 
kinds  of  cells,  but  the  duties  per- 
formed by  each  are  different.  For  ex- 
ample, a  glance  at  Fig.  14  will  show 
that  in  the  inner  layer  certain  colum- 
nar cells  exist,  provided  with  a  fla- 
gellum  and  encircling  collar,  the  ap- 
pearance being  strikingly  like  certain 
of  the  Protozoa  (Fig.  6,  B).  During 
life  their  whip-like  processes,  lashing 
backward  and  forward  in  perfect  uni- 
son, produce  currents  of  water  which 
continually  pass  through  the  body. 
The  food  thus  entering  the  animal  is 
taken  up  by  the  cells  of  the  inner 
layer  as  it  passes  by.  The  supply, 
however,  is  usually  more  than  suffi- 
cient to  meet  the  demands  of  this 
layer,  and  the  excess  is  passed  on  to 
the  middle  and  outer  layers.  The 
exact  method  by  which  this  occurs  is  still  a  matter  of 
doubt,  but  there  seems  to  be  little  question  but  that 
each  cell  of  the  body  receives  its  food  in  a  practically  un- 
modified condition,  requiring  that  it  digest  as  well  as 
assimilate.  The  oxygen  necessary  to  this  latter  process 


FIG.  13.— One  of  the  sim- 
plest sponges  (Calcolyn- 
thus  primigenius  (after 
HAECKEL).  A  portion 
of  the  wall  has  been  re- 
moved to  show  the  in- 
side. 


ANIMAL  FORMS 


is  absorbed  by  all  parts  of  the  body  in  contact  with  the 

water. 

27.  Skeleton  of  sponges. — When  it  is  remembered  that 

the  protoplasm  composing  the  cells  of  the  sponge  has  about 

the  same  consistence 
as  the  white  of  egg, 
it  will  be  readily  un- 
derstood why  the 
greater  number  of 
sponges  possess  a  skel- 
eton. Without  such 
a  support  the  larger 
globular  or  branched 
forms  could  not  ex- 
ist, and  even  in  the 

smaller  members  there  would  be  danger  of  a  collapse  of  the 

body  walls  and  consequent  stoppage  of  the  food  supply, 

owing  to  the  closure  of  the  pores.     So  in  all  but  a  very  few 

thin  or  fiat  forms  a  skeleton  appears  in  the  young  sponge 

almost    before    growth 

has    fairly   begun,   and 

this  increases  with  the 

body  in  size  and  com- 
plexity.     It   is  formed 

by  the   activity  of  the 

cells  of  the  middle  layer, 

and   may  be  composed 

either  of  a  lime   com- 


FIG.  14.— Portion  of  wall  of  sponge,  showing  three 
layers,  e,  outer  layer  ;  i,  inner  layer,  consisting 
of  collared  cells  ;  m,  middle  layer,  consisting  of 
irregular  cells,  among  which  are  the  radiate  spic- 
ules  and  egg-cells. 


FIG.  15.— Different  types  of  sponge  spicules. 


pound  resembling  mar- 
ble, or  of  flint,  or  of  a 
horn-like  substance  resembling  silk,  or  these  may  exist  in 
combination  in  certain  species.  When  consisting  of  either 
of  the  first-named  substances  it  is  never  formed  in  one 
continuous  piece,  but  of  a  vast  multitude  of  variously  shaped 
crystal-like  bodies  termed  spicules  (Fig.  15).  These  occur 
everywhere  throughout  the  body,  firmly  bound  together 


THE  SPONGES  27 

by  means  of  cells,  or  so  interlocked  that  they  form  a  rigid 
support  to  which  the  fleshy  substance  is  bound  and  through 
which  the  numerous  canals  penetrate. 

In  a  relatively  few  species  only  does  the  skeleton  con- 
sist of  horn,  though  there  are  many  in  which  horn  and  flint 
exist  together.  In  the  former  event,  if  the  skeleton  be 
elastic  and  of  sufficient  size,  it  becomes  valuable  to  others 
than  the  naturalist,  for  the  familiar  sponges  of  commerce 
are  the  horny  skeletons  of  forms  usually  taken  in  the  West 
Indies  or  in  the  Mediterranean  Sea.  In  these  localities  the 
animals  are  pulled  off  by  divers,  or  with  hooks,  and  are  then 
spread  out  in  shallow  water  where  the  protoplasmic  sub- 
stance rapidly  decays.  The  remaining  skeleton,  thoroughly 
washed  and  dried,  is  ready  for  the  markets  of  the  civilized 
world. 

Examining  a  bit  of  such  a  "  sponge  "  under  a  magnify- 
ing glass,  it  will  be  seen  that  the  skeleton  is  not  composed 
of  various  pieces,  but  of  one  continuous  mass  of  branching 
fibers,  which  interlace  and  unite  in  apparently  the  greatest 
confusion ;  yet  in  the  living  animal  these  were  perfectly 
adapted  to  the  position  of  the  canals  and  the  general  needs 
of  the  animal. 

Besides  being  a  scaffold-work  to  which  the  fleshy  portions 
of  the  body  are  fastened,  the  skeleton  serves  also  for  pro- 
tection. In  some  species,  needle-like  spicules  as  fast  as 
they  are  formed  are  partly  pushed  out  over  the  entire  sur- 
face of  the  body,  giving  the  appearance  of  a  spiny  cactus ; 
or  in  other  cases  they  are  arranged  in  tufts  about  the  canals, 
effectually  preventing  the  entrance  of  any  marauder. 
Thus  perfectly  protected,  the  sponges  have  but  few  natural 
enemies,  and  hence  it  is  that  in  favorable  localities  they 
grow  in  great  profusion. 

28.  Race  histories  and  life  histories.— We  have  now  traced 
living  things  from  their  simplest  beginnings,  where  they 
exist  as  single  cells,  and  have  seen  that  in  bygone  times 
similar  forms  have  united  into  simple  colonies,  and  these 


28  ANIMAL  FORMS 

through  a  division  of  labor  among  the  constituent  cells 
have  resulted  in  Volvox-like  colonies.  There  are  the  strong- 
est reasons  for  the  belief  that  as  these  simple  forms  scat- 
tered into  various  surroundings  and  underwent  changes  to 
meet  the  shifting  conditions,  they  assumed  different  de- 
grees of  complexity  that  have  resulted  in  the  animal  forms 
of  the  present  day. 

It  may  have  been  noticed  also  that  the  sponge  in  its 
development  passes  through  these  stages :  a  single-celled 
egg ;  later,  a  young  form  similar  to  Pandorina,  then  growing 
to  look  like  Volvox,  and  finally  assuming  its  permanent  form. 
The  history  of  the  race  of  sponges  and  their  development 
through  a  long  line  of  ancestry  of  increasing  complexity  is 
thus  told  by  the  sponge  as  it  develops  from  the  egg  into 
the  adult ;  and,  so  far  as  we  know,  all  the  many-celled  ani- 
mals in  their  growth  from  the  egg  repeat  more  or  less 
clearly  the  stages  passed  through  by  their  forefathers. 


CHAPTER  V 

THE   CCELENTBRATES 

29.  General  remarks,— This  division  of  the  many-celled 
animals  includes  the  jelly-fishes,  sea-anemones,  and  corals. 
A  few  species  live  in  fresh  water,  but  the  majority  are  con- 
fined to  the  sea,  being  found  everywhere  from  the  shore- 
line and  ocean   surface  to  the    most    profound    depths. 
Adapted  to  different  surroundings  and  modes  of  life,  they 
constitute  a  vast  assemblage  of  the  most  bewildering  di- 
versity.    In  some  cases  their  resemblance  to  plants  is  re- 
markable, and  the  term  zoophyte  or  "  plant  animal,"  occa- 
sionally applied  to  them,  is  the  relic  of  former  times  when 
naturalists  confounded  them  with  plants.     Even  to-day 
certain  species  are  sometimes  collected  and  preserved  as 
seaweeds  by  the  uninformed. 

The  general  plan  on  which  all  coelenterates  are  con- 
structed is  a  simple  sac,  in  some  respects  resembling  that 
of  the  lower  sponges,  yet,  since  the  modes  of  life  of  the 
members  of  the  two  groups  are  usually  quite  unlike,  we 
shall  find  many  profound  differences  between  them. 

30.  The  fresh-water  Hydra. — The  bodily  plan  comes  out 
most  clearly  in  the  Hydra  (Fig.  16,  A,  D),  which  occurs 
upon  the  stems  and  leaves  of  submerged  fresh-water  plants 
in  this  and  other  countries.    Its  body,  of  a  green  or  grayish 
color,  according  to  the  species,  scarcely  ever  attains  a  diam- 
eter greater  than  that  of  an  ordinary  pin  nor  a  length  ex- 
ceeding half  an  inch.     One  end  of  the  cylindrical  organism 
is  attached  to  some  foreign  object  by  means  of  a  sticky 
secretion,  but  as  occasion  requires  it  may  free  itself,  and  by 

29 


30 


ANIMAL  FORMS 


means  of  a  "  measuring-worm  "  movement  travel  to  another 
place. 

Examined  under  a  hand  lens,  the  free  end  of  the  body 
will  be  found  to  support  six  to  eight  prolongations  known 

as  tentacles,  which 
serve  to  convey 
food  to  the  mouth, 
centrally  located 
in  their  midst. 
This  opening,  un- 
like that  of  the 
sponges,  is  the 
only  one  leading 
directly  into  the 
large  central  gas- 
tric cavity  which 
occupies  nearly 
the  entire  animal 
(Fig.  16,  D).(  As 
in  the  sponge,  the 
cells  of  the  body 
are  arranged  in 
the  form  of  defi- 
nite layers,  but  the 
middle  one  is  rep- 
resented only  by 
a  thin  gelatinous 
sheet. 

31.  Organs  of 
defense.  --  These 
are  the  so-called 
lasso  or  nettle-cells 
(Fig.  16,  C).  Some 
of  the  cells  of  the  outer  layer  possess,  in  addition  to  the 
elements  of  the  typical  cell,  a  relatively  large  ovoid  sac 
filled  with  a  fluid,  and  also  a  spirally  wound  hollow  thread 


FIG.  16.— The  fresh-water  Hydra.  A,  entire  animal,  de- 
veloping a  new  individual  (enlarged  25  times).  B,  C, 
nettle-cells  (after  SCHNEIDER)  ;  D,  section  through 
the  body. 


THE  CCELENTERATBS  31 

provided  with  barbs  near  its  base.  On  the  outer  extremity 
of  the  nettle-cell  projects  a  delicate  bristle-like  process,  the 
trigger  hair.  These  cells  are  especially  abundant  on  the 
tentacles  (Fig.  16,  A,  D),  forming  close,  knob-like  eleva- 
tions or  "  batteries,"  thus  rendering  it  practically  impossi- 
ble for  any  free-swimming  organism  to  avoid  touching  them 
in  brushing  against  the  tentacles.  In  such  an  event  the  dis- 
turbances conveyed  through  the  trigger  hair  set  up  in  some 
unknown  way  very  rapid  changes  in  the  cell.  This  causes 
the  sac  to  discharge  the  coiled  thread  and  barbs  into  the 
body  of  the  intruder,  which  is  rendered  helpless  by  the  par- 
alyzing action  of  the  fluid  conveyed  through  the  thread. 
Thus  benumbed  it  is  rapidly  borne  to  the  mouth  and  swal- 
lowed. In  time  new  nettle-cells  develop  to  take  the  place 
of  those  discharged  and  consequently  worthless. 

32.  Digestion  of  food. — Upon  the  interior  of  the  body  of 
Hydra  and  all  of  the  coelenterates  the  food,  by  reason  of  its 
large  size,  is  incapable  of  being  taken  into  the  various  cells. 
It  is  necessary,  therefore,  to  break  it  up  into  smaller  masses, 
and  this  is  accomplished  through  the  solvent  action  of  the 
digestive  fluid  poured  over  it  from  some  of  the  cells  of  the 
adjacent  inner  layer.     When  subdivided,  the  granules  swept 
about  the  gastric  cavity  by  the  beating  of  the  flagella  (Fig. 
16,  D)  are  seized  by  the  processes  on  the  free  surfaces  of  the 
remaining  inner  layer  cells,  where  they  undergo  the  final 
stages  of  -digestion  ;  then  in  a  dissolved  state  they  become 
absorbed  and  assimilated  by  all  the  cells  of  the  body. 

33.  Methods  of  multiplication. — Very  frequently,  espe- 
cially if  the  Hydra  has  been  well  fed,  two  or  three  pro- 
cesses arising  as   outpushings  of  the  body  wall   may  be 
noted  upon  the  sides  of  the  animal  (Fig.  16,  A,  D).    If 
these  be  watched  from  time  to  time  they  are  found  to  in- 
crease in  size,  and  finally,  upon  their  free  extremities,  to 
develop  a  mouth  and  surrounding  tentacles.    Up  to  this 
point  growth  has  taken  place  as  a  result  of  the  assimilation 
of  nutritive  substances  supplied  from  the  parent ;  but  a  con- 

25 


32 


ANIMAL  FORMS 


striction  soon  occurs  which 
separates  the  young  from  the 
parent,  and  from  that  time 
on  the  two  lead  independent 
existences.  At  other  times 
this  asexual  method  of  mul- 
tiplication is  replaced  by  sex- 
ual reproduction,  where  new 
individuals  arise  from  fertil- 
ized eggs.  Both  eggs  and 
sperm  arise  in  Hydra  and  in 
some  other  animals  in  the 
same  individual,  but  in  all 
such  cases  the  eggs  are  fertil- 
ized by  sperm  which  escape 
from  some  other  individual. 
The  fertilized  egg,  surround- 
ed by  a  firm  coat,  separates 
from  the  parent,  drops  to  the 
bottom,  and  after  a  period  of 
rest  develops  into  a  little  Hy- 
dra which  hatches  and  enters 
upon  a  free  existence. 

FIG.  17.— Different  types  of  Hydrozoan 
colonies.  From  Nature,  the  lower 
species  magnified  about  50  diameters. 


THE  CCELENTERATES  33 

34.  Hydrozoa,  or  Hydra-like  animals.— Attention  has  al- 
ready been  directed  to  the  fact  that  the  structure  of  Hydra 
is  the  simplest  of  the  coelenterates  ;  nevertheless,  the  thou- 
sand or  more  species  belonging  to  this  class  which  present 
a  much  more  complicated  appearance  (Fig.   17)  possess 
many  fundamental  Hydra-like  characters.     It  is  owing  to 
this  fact  that  this  assemblage  of  forms  has  been  placed  in 
the  class  of  the  Hydrozoa,  or  Hydra-like  animals. 

With  but  very  few  exceptions  the  members  of  this  class 
are  marine,  usually  living  near  the  shore-line,  where  at 
times  their  plant-like  bodies  occur  in  the  greatest  profusion 
attached  to  rocks,  seaweeds,  or  the  bodies  of  other  animals, 
particularly  snails  and  crabs.  Fig.  17  (upper  colony)  gives 
a  good  idea  of  one  of  the  more  complex  forms,  whose  tree- 
like body  attains  in  some  cases  the  relatively  giant  height  of 
from  15  to  25  c.m.  (six  to  ten  inches).  In  early  life  it  bears 
a  close  resemblance  to  a  Hydra.  Buds  form  in  much  the 
same  way,  but  they  retain  permanently  their  connection  with 
the  parent,  and  in  turn  bear  other  buds,  until  finally  the  form 
shown  in  the  figure  is  attained.  In  the  meantime  root-like 
processes  have  been  forming  which  afford  firm  attachment 
to  the  object  upon  which  the  body  rests.  Also  during  this 
process  the  cells  of  the  outer  layer  form  a  horny  external 
skeleton  ensheathing  the  entire  organism  except  the  ter- 
minal portions  (the  hydranths,  Fig.  18,  B)  bearing  the  ten- 
tacles. The  gastric  cavities  of  all  communicate,  and  the 
food  captured  by  one  ministers  in  part  to  its  own  needs 
and,  swept  through  the  tubular  stalks  and  roots,  is  also 
shared  by  all  other  members. 

35.  Jelly-fishes  and  the  part  they  play.— During  the  pro- 
cess of  growth  a  number  of  stubby  branches  arise  which 
differ  from  the  ordinary  type  in  shape,  and  also  in  many 
cases  as  regards  color.  .  These  club-like,  fleshy  portions  de- 
velop close-set  buds  (Fig.  18,  c)  which  early  assume  a  bell- 
like  shape,  the  point  of  attachment  corresponding  to  the 
handle,  while  the  clapper  is  represented  by  a  short,  slender 


34  ANIMAL  FORMS 

process  bearing  on  its  end  an  opening  which  becomes  the 
mouth  (Fig.  18,  A).  Around  the  margin  of  the  bell  nu- 
merous tentacles  develop,  and  at  the  same  time  the  gelati- 
nous substance  situated  between  the  outer  ^and  inner  layers 
of  the  bell  expands  to  a  relatively  enormous  degree,  giving 
it  an  increasing  globular  form  and  glassy  appearance. 


B 


FIG.  18.— A  jelly-fish  (Gonionemus),  slightly  enlarged.  The  stalked  mouth  is  shown 
in  dotted  outline.  B,  C,  enlarged  portions  of  a  hydroid  colony  bearing  the 
mouth  and  tentacles  ;  j,  a  capsule  within  which  the  jelly-fish  develop  ;  D,  dia- 
gram of  jelly-fish,  illustrating  its  method  of  locomotion. 

Finally,  vigorous  movements  rupture  the  connection  with 
the  parent,  and  this  newly  developed  outgrowth,  usually  ' 
small,  becomes  an  independent  organism  popularly  termed 
a  jelly-fish.    While  the  external  form  of  the  jelly-fish  appears 
to  be  widely  different  from  the  hydranths,  a  more  careful 
study  shows  the  difference  to  be  superficial.     Some  zoolo- 
gists believe  that  jelly-fishes  are  simply  buds  which  have 
become  fitted  to  separate  and  swim  away  from  the  colony   ' 
in  order  to  distribute  the  young,  as  described  hereafter. 
When  the  stalked  colonies  are  very  abundant  the  jelly- 


THE  (XELENTERATES  35 

fishes  may  be  liberated  in  such  multitudes  that  the  upper 
surface  of  the  ocean  for  many  miles  may  be  closely  packed 
with  them  in  numbers  reaching  far  into  the  millions.  In 
these  positions  they  are  carried  both  by  oceanic  currents 
and  through  the  alternate  expansion  and  contraction  of  the 
bell,  a  movement  resembling  the  partial  closing  and  open- 
ing of  an  umbrella.  In  the  jelly-fish  the  contraction  is 
more  vigorous  and  rapid  than  the  opening  in  the  velum  or 
veil  (Fig.  18,  b)  which  is  so  narrowed  that  the  water  in 
the  subumbrella  space  (a)  is  driven  through  it  with  con- 
siderable force,  which  results  in  driving  the  body  in  the 
opposite  direction. 

The  life  of  a  jelly-fish  is  perhaps  of  short  duration,  last- 
ing not  more  than  a  few  hours  in  some  species,  up  to  two 
or  three  weeks  in  others,  but  during  that  period  they  pro- 
duce multitudes  of  eggs  which  develop  into  minute  free- 
swimming  young.  These  settle  down  on  some  rock  or  sea- 
weed, and  soon  develop  a  Hydra-like  body  which,  after  the 
fashion  described  above,  grows  into  another  tree-like  colony. 

36.  Alternation  of  generations.— It  will  be  noticed  that 
the  offspring  of  the  jelly-fishes  are  not  jelly-fishes,  but  stalked 
colonies,  and  these  latter  forms  give   rise  to  jelly-fishes. 
This  is  known  as  the  alternation  of  generations,  the  jelly- 
fish generation  alternating  with  the  colonial  form.     This 
characteristic  is  of  the  greatest  service  in  preventing  the 
extermination   of   the  race.     Were   the  stalked  forms  to 
give  rise  directly  to  other  stationary  colonies,  it  is  obvious 
that  before  long  all  the  available  space  in  the  immediate 
locality  would  be  filled.      The   food   supply,  always  lim- 
ited, would  not  suffice,  and  starvation  of  some  or  imper- 
fect development  of  all  would  result ;  but  by  means  of  the 
free-swimming  jelly-fish  new  colonies  are  established  over 
very  extensive  areas,  and  favorable  situations  are   held 
by  all. 

37.  More  complex  types. — As  mentioned  above,  there  are 
perhaps  upward  of  a  thousand  species  of  Hydrozoa,  all  with 


36 


ANIMAL  FORMS 


essentially  the  same  structure  but  with  various  modes  of 
branching  (for  some  of  the  commoner  modes,  see  Fig.  17)- 
In  some  of  the  higher  forms  a  division  of  labor  has  arisen 
among  various  members  of  the  association  which  has  led  to 
most  interesting  results.  For  example,  Fig.  19  represents 
a  species  of  hydroid  found  investing  the  shells  of  sea-snails 
occupied  by  hermit  crabs  (Fig.  60).  To  the  unaided  eye 
its  appearance  is  that  of  a  delicate  vegetable  growth,  but 
when  placed  under  the  microscope  it  is  found  to  consist  of 

a  multitude  of  Hydra-like 
animals  united  by  a  hollow 
branching  root  system  con- 
necting the  gastric  cavities 
of  all  of  them  (Fig.  19). 
Certain  individuals  (a) 
with  tentacles  and  a  mouth 
resemble  a  Hydra ;  others, 
without  a  mouth  and  ten- 
tacles, are  reduced  to  a 
club-like  form  (b)  liberally 
supplied  with  nettle-cells 
upon  their  free  extremi- 
ties; while  the  third  type 
(c),  likewise  devoid  of  a  mouth,  possesses  rudiments  of  ten- 
tacles below  which  are  borne  numerous  clumps  of  repro- 
ductive cells.  The  first  type,  the  only  one  possessing  a 
mouth,  captures  the  food,  and  after  digesting  it  distributes 
the  greater  portion  to  the  remaining  members  by  means  of 
the  connecting  root  system ;  those  of  the  second  form,  de- 
fending the  others  by  means  of  their  nettle-cells  against 
the  inroads  of  a  foreign  enemy,  are  the  soldiers  of  the  colo- 
ny; while  the  third  type  produces  the  eggs  from  which 
new  individuals  develop. 

In  some  of  the  higher  Hydrozoa,  the  Portuguese  man- 
of-war  (Fig.  20),  this  division  of  labor  has  reached  a  more" 
advanced  stage  of  development,  and  in  addition  the  entire 


FIG.  19.— An  enlarged  portion  of  a  hydroid 
colony  (Hydractinia),  showing  (a)  the 
nutritive  polyp,  (5)  the  defensive  polyp, 
and  (c)  the  reproductive  polyp. 


THE  CCELENTERATES 


37 


colony  is  fitted  for  a  free-swimming  existence.  What  cor- 
responds ordinarily  to  the  attached  stalk  in  other  forms 
terminates  in  a  bladder-like  expansion,  distended  with 
gas,  that  serves  as  a  float.  From  it  are  suspended  individ- 
uals resembling  great  stream- 
ers sometimes  many  feet  in 
length,  without  mouths,  but 
loaded  with  nettle-cells  that 
enable  them  to  capture  the 
food,  which  is  conveyed  to  the 
second  type,  the  nutritive 
polyps.  Each  of  these  is  a 
simple  tube  bearing  a  mouth, 
and  within  them  the  food  is 
digested  and  distributed  by 
means  of  a  branching  gastric 
cavity  extending  throughout 
the  entire  colony.  Then  there 
are  individuals  like  mouthless 
jelly-fishes  which  bear  the 
eggs  and  care  for  the  perpet- 
uation of  the  colony ;  and  be- 
sides these  there  may  be  some 
whose  duty  it  is  to  defend  the 
rest,  and  others  whose  active 
swimming  movements,  to- 
gether with  the  wind,  drive 
the  colony  about.  Thus  uni- 
ted, sharing  the  food  supply 
and  working  for  the  general  welfare  of  all,  the  members  of 
this  colony  live  in  greater  security  and  with  less  effort  than 
if,  as  separate  individuals,  each  was  fighting  the  battles  of 
life  alone. 

38.  Scyphozoa, — The  greater  number  of  the  larger  and 
more  conspicuous  jelly-fishes  are  included  under  this  term. 
In  general  shape  and  locomotion  they  resemble  those  of  the 


PIG.  20.— A  colonial  jelly-fish  (Physalia). 
From  Nature. 


38 


ANIMAL  FORMS 


preceding  group  (Fig.  21),  but,  while  the  latter  are  generally 
very  small,  these  forms  are  commonly  from  four  to  twelve 
inches  in  diameter,  and  some  measure  one  to  two  meters 
(three  to  six  feet)  across  the  bell.  They  are  also  distin- 
guished by  means  of  tentacles  which  extend  from  the  cor- 
ners of  the  mouth  sometimes  to  a  distance  of  several  feet, 

and  together  with  the 
marginal  tentacles  are 
formidable  weapons  for 
capturing  small  crabs, 
fishes,  and  other  ani- 
mals which  serve  as 
food.  In  turn  these 
forms  serve  as  the  food 
of  many  whales,  por- 
poises, and  numerous 
fishes  which  hunt  them 
down,  though  the 
amount  of  nourishment 
they  contain  is  prob-j 
ably  relatively  small 
owing  to  the  fact  that 
in  their  composition 
there  is  a  large  percent- 
age of  water  (99  per 
cent  in  some  species).  The  lobed  margin  of  the  bell,  the 
absence  of  a  definite  swimming  organ  or  velum,  and  the 
character  of  several  of  the  internal  organs,  distinguish  the 
larger  from  the  smaller  jelly-fish  ;  but  the  greatest  differ- 
ence, however,  is  in  the  method  of  development. 

39.  Development. — The  eggs  arise  from  the  inner  layer 
of  the  jelly-fish  and  drop  into  the  gastric  cavity,  where  each 
develops  into  a  ciliated  two-layered  sac  in  some  respects 
like  that  of  a  young  sponge.  Swimming  away  from  the 
parent,  they  finally  settle  down,  and  attaching  themselves 
(Fig.  22,  a)  assume  the  external  form  and  habits  of  the  sea- 


FIG.  21.— A  jelly-fish  (Rhizostoma),  about  one- 
fourth  natural  size. 


THE  CCELENTERATES 


39 


anemones,  described  in  the  next  section.     In  the  course  of 
time  remarkable  changes  ensue,  which  first  manifest  them- 


FIG.  22.— Stages  in  the  development  of  a  scyphozoan  jelly-fish,  a,  the  attached 
young,  which  in  b  has  separated  into  a  number  of  disks,  each  of  which  becomes  a 
jelly-fish,  c. — After  KORSCHELT  and  HEIDEB. 

selves  in  a  series  of  grooves  encircling  the  body.  These 
grow  deeper,  and  the  body  of  the  animal  finally  comes  to 
resemble  a  pile  of  sau- 
cers with  the  edge  of 
each  developed  into  a 
number  of  lobes  (Fig. 
22,  I).  One  after  an- 
other each  saucer,  to 
preserve  the  simile, 
raises  itself  from  the 
top  of  the  pile  and 
swims  away,  and  is 
clearly  seen  to  be  a 
jelly-fish,  though  con- 
siderably unlike  the 
adult.  As  growth  pro- 
ceeds, however,  it  un- 
dergoes a  series  of  transformations  which  result  in  the 
adult  form. 


FIG.  23.— An  attached  scyphozoan  jelly-fish 
(Halidystus).    Natural  size,  from  Nature. 


40 


ANIMAL   FORMS 


40.  Sea-anemones. — In  its  external  appearance  the  sea- 
anemone  (Fig.  24)  bears  some  resemblance  to  the  Hydra,  but 
is  of  a  much  larger  size  (1  to  45  c.m.,  or  \  inch  to  1-J  feet 
in  diameter),  and  is  frequently  brilliantly  colored.  The 
number  of  tentacles  is  also  more  numerous,  and  the  mouth 
leads  into  the  body  by  means  of  a  slender  esophagus  (Fig. 
25).  Numerous  partitions  from  the  body  wall  extend  in- 
ward, and  many  unite  to  the  esophagus,  keeping  the  latter 


FIG.  24.— Sea  anemones  (the  two  upper  figures)  and  solitary  coral  polyps. 

in  position.  Below  the  esophagus  each  partition  projects 
into  the  great  cavity  of  the  body  and  bears  upon  its  inner 
free  edge  several  important  structures.  The  first  of  these, 
known  as  the  mesenteric  filaments  (Fig.  25),  appearing  like 
delicate  frills,  plays  an  active  part  in  the  digestion  of  the 
food.  Associated  with  these  are  long,  slender  threads, 


THE  CCELENTERATES 


41 


closely  packed  with  innumerable  lasso-cells,  which  may  be 
thrown  out  through  openings  in  the  body  wall  when  the 
animal  is  attacked.  Lasso-cells  are  also  very  numerous  on 
the  tentacles,  which  are  thus  to  some  extent  defensive,  but 
are  chiefly  active  in  capturing  the  crabs  and  small  fish 
which  serve  as  food. 

The  partitions  also  carry  eggs  which  may  undergo  the 
first  stages  of  their  growth  within  the  body,  and  when 
finally  able  to  swim 
are  sent  out  through 
the  mouth  opening 
by  hundreds  to  seek 
out  favorable  situa- 
tions, there  to  set- 
tle down  and  re- 
main. In  some  spe- 
cies the  young  may 
sometimes  arise  as 
buds,  as  in  Hydra 
(Fig.  24),  and  in 
others  the  animals 
have  been  described 
as  splitting  longi- 
tudinally into  two 
equal -sized  young. 

41.  Corals.— The 
coral    polyps    also 

belong  to  this  group,  showing  a  very  close  resemblance  to 
the  sea-anemones.  In  most  cases  they  develop  a  firm  skel- 
eton of  lime,  commonly  known  as  "  coral,"  which  serves  to 
protect  and  support  the  body.  In  a  few  species  the  polyps 
throughout  life  are  solitary,  and  with  skeleton  comparative- 
ly simple  (Fig.  24) ;  but  the  larger  number  of  species  be- 
come more  complex  by  developing  buds,  which  retain  their 
connection  with  the  parent,  and  in  turn  produce  other  out- 
growths with  the  ultimate  result  that  highly  branched 


FIG.  25.— Longitudinal  section  through  the  body  of  a 
sea-anemone,  oe,  esophagus  ;  m.  /.,  mesenterial 
filaments  ;  r.,  reproductive  organs. 


42  ANIMAL  FORMS 

colonies  are  produced  (Fig.  26).  At  the  same  time  the 
outer  layer  of  the  body  is  continually  forming  a  skeleton 
which  encloses  the  colony  as  a  sheath,  except  at  the  ter- 
mination of  each  branch,  where  the  mouth  and  tentacles 
are  located.  In  certain  species — for  example,  the  sea  pens 
(Pennatuld)  and  sea  fans  (Gorgonia)—&  skeleton  may  be 


FIG.  26.— Small  portions  of  coral  colonies,  with  some  of  the  polyps  expanded. 

formed  of  myriads  of  lime  spicules,  somewhat  like  those 
of  the  sponge,  which  are  bound  together  by  the  fleshy 
substance  of  the  body;  but  the  skeleton  of  most  of  the 
common  forms  in  the  ocean,  and  the  coral  found  in 
general  collections,  is  stony.  According  to  their  method 
of  branching,  such  specimens  have  received  various  popu- 
lar names,  such  as  brain,  stag-horn,  organ-pipe,  and  fun- 
gous corals. 


THE  CCELENTERATES  43 

Nearly  all  species,  like  the  sea-anemones,  are  brilliantly 
colored  during  life,  and  several  are  highly  phosphorescent. 
All  are  marine,  and  while  they  are  found  everywhere,  from 
the  shore-line  to  great  depths,  the  more  abundant  and 
larger  species  inhabit  the  clear,  warm  waters  of  the  tropics 
down  to  a  depth  of  one  hundred  and  sixty  feet.  In  such 
regions  the  stag-horn  corals  especially  grow  in  the  wildest 
profusion,  and  become  tall  and  greatly  branched.  Except 
in  quiet  water  they  are  continually  being  broken  by  the 
waves,  beaten  into  fragments,  and  the  resulting  sand  is 
deposited  about  their  bases.  As  a  result  of  this  continu- 
ous growth  and  erosion,  there  have  been  formed  from  coral 
sand  mixed  with  the  shells  of  mollusks  and  the  skeletons 
of  various  Protozoa  several  of  the  islands  along  the  Florida 
coast  and  many  of  those  of  the  Pacific,  some  of  them 
hundreds  of  miles  in  extent. 


CHAPTER  VI 

«  ' 

THE    WORMS 

4>  General  Characteristics.— The  bodies  of  the  animals 
comprising  the  two  preceding  groups  are  exposed  on  all 
sides  equally  to  the  water  in  which  they  live  and  are  radi- 
ally symmetrical ;  but  in  the  worms,  one  side  of  the  body 
is  fitted  for  creeping,  and  for  the  first  time  we  note  a  well- 
marked  dorsal  (back)  and  ventral  (under)  surface.  In  the 
former,  the  body,  like  a  cylinder,  may  be  divided  into  simi- 
lar halves  by  any  number  of  planes  passing  lengthwise 
through  the  middle ;  but  in  the  worms,  the  right  and  left 
halves  only  are  exposed  equally  to  their  surroundings,  and 
there  is,  accordingly,  only  one  plane  which  divides  the  body 
into  corresponding  halves,  so  that  these  animals,  like  all 
higher  forms,  are  bilaterally  symmetrical.  In  creeping,  also, 
one  end  of  the  body  is  directed  forward  and  it  thus  be- 
comes correspondingly  modified.  It  usually  bears  the 
mouth,  and  may  be  provided  with  eyes,  feelers,  or  organs 
of  touch,  and  various  other  structures  which  enable  the 
worm  to  recognize  the  nature  of  its  surroundings.  The 
nervous  and  muscular  systems  are  better  developed  than  in 
the  foregoing  groups,  and  we  note  a  greater  vigor  and  defi- 
niteness  in  the  animal's  movements,  and  in  various  ways  the 
worms  appear  better  able  to  avoid  or  ward  off  their  enemies, 
recognize  and  select  their  food,  and  in  general  adapt  them- 
selves to  the  conditions  of  life. 

The  division  of  the  worms  is  a  very  large  one,  and  in 
some  respects  difficult  to  define,  owing  to  the  close  resem- 
44 


THE  WORMS 


45 


blance  which  many  of  them  show  to  animals  in  other 
groups.  All  the  invertebrates,  therefore,  except  the  crabs 
and  insects,  were  placed  in  one  group  until  subsequent 
study  made  it  possible  to  classify  them  more  exactly.  Ac- 
cording to  the  general  shape  of  the  body,  and  the  arrange- 
ment of  internal  organs,  worms  are  divided  into  a  number 
of  groups,  chief  among  which  are  the  flatworms,  the  thread 
or  roundworms,  and  the  ringed  worms  or  annelids. 

THE  FLATWORMS 

43.  Form  and  habitat. — The  flatworms,  as  their  -me 
indicates,  are  much  flattened,  leaf-like  forms,  some  species 
living  in  damp  places  on  land, 
in  fresh  -  water  streams  or 
ponds,  or  along  the  seacoast, 
while  a  variety  of  other  spe- 
cies are  parasitic.  The  free 
forms  (Fig.  27)  are  usually 
small,  barely  reaching  a  length 
greater  than  five  or  seven  cen- 
timeters (2  to  3  inches),  but 
some  of  the  parasitic  species 
(Fig.  31)  attain  the  great 
length  of  six  to  thirteen  me- 
ters (20  to  40  feet). 

The  free-living  forms  usu- 
ally occur  on  the  under  side 
of  stones,  and  frequently  are 
so  delicate  that  a  touch  is 
sufficient  to  destroy  them.  A 
few  species  are  almost  trans-  FIG.  ST.-A,  fre8h-water  natworm  (Pto- 

.  naria) ;   B,  marine  flatworm  (Lepto- 

parent,    While    many    are    COl-          piana}.    Enlarged,  from  Nature. 

ored  to  harmonize  completely 

with  their  surroundings,  so  that,  even  though  fragile  and 
defenseless,  they  escape  the  attacks  of  enemies  by  being 
overlooked.  The  night-time  or  dark  days  are  their  hunting 


46 


ANIMAL  FORMS 


season,  and  at  such  periods  they  may  be  found  moving  about 
with  a  steady  gliding  motion  (due  to  cilia  covering  the  en- 
tire body),  varied  occasionally  by  a  looping,  caterpillar  move- 
ment, or  by  swimming  with  a  flapping  of  the  sides  of  the 
body.  When  watched  at  such  times  they  may  sometimes 
be  seen  to  snatch  up  small  worms,  snails,  small  crabs  and 
insects,  which  serve  as  food. 

More  closely  examining  one  of  these  forms,  for  example, 
the  species  usually  found  on  the  under  side  of  sticks  and 
stones  in  our  shallow  fresh-water  streams  (Fig.  27,  A),  we  note 
that  the  forward  end  is  not  developed  into  a  well-defined 

head  as  in  the  higher  worms, 
but  is  readily  determined  by 
the  presence  of  very  simple 
eyes  and  tentacles,  while  the 
lower  creeping  surface  is  dis- 
tinguished by  a  lighter  color 
and  the  presence  of  the 
mouth.  Through  this  small 
opening  a  slender  .proboscis 


m 


(in  reality  the  pharynx)  may 
be  extended  some  distance, 
and  may  be  seen  to  hold  the 
small  organisms  upon  which 
it  lives  until  they  are  suffi- 
ciently digested  to  be  taken 
into  the  body. 

44    Digestive  system.— In 
the  smaller  flatworms,  some 
FIG.  28.-Anatomy  of  fresh-water  flat-    of  wnich  are  scarcely  larger 

worm  (Planaria).  exs,  excretory  sys-     than   many    of    the    Protozoa 

rnCLTr^.  Toet    the  ^entary  canal  is  a  sim- 
ous  system.  pie  unbranched  tube ;  but  in 

the  larger  forms  such  an  ap- 
paratus is  replaced  by  a  greatly  branched  digestive  tract 
which  furnishes  an  extensive  surface  for  the  rapid  absorp- 


THE   WORMS 


47 


tion  of  food,  and  extending  deep  into  the  tissues  of  the 
body,  carries  nutriment  to  otherwise  isolated  regions.  In 
the  fresh-water  forms  and  their  allies  there  are  three  main 
branches  of  the  intestine  (Fig.  28),  while  in  many  of  those 
from  the  sea  there  are  several,  and  their  arrangement 
affords  a  basis  for  their  general  classification. 

45.  Excretory  system. — In  the  sponges  and  coelenterates 
the  wastes  are  cast  out  by  the  various  cells  into  the  gastric 
cavity  or  at  once  to  the  exterior  with- 
out the  aid  of  any  pronounced  system 

of  vessels;  but  in  the  flatworms  sev- 
eral of  the  organs  are  deeply  buried 
within  the  tissues  of  the  body  and  a 
drainage  system  becomes  a  necessity. 
This  consists  of  a  paired  system  of  ves- 
sels extending  the  length  of  the  ani- 
mal (Fig.  28)  and  provided  with  numer- 
ous branches,  some  of  which  open  at 
various  points  on  the  surface  of  the 
body,  while  the  others  terminate  in 
spaces  (Fig.  29,  s)  among  the  organs  in 
what  are  known  as  flame-cells.  The 
substances  which  accumulate  in  these 
spaces  are  gathered  up  by  the  flame- 
cell,  poured  into  the  space  it  contains,  and  by  means  of  the 
vibratory  motion  of  its  flagellum  (/),  a  movement  bearing 
a  fancied  resemblance  to  the  flickering  of  a  flame  in  the 
wind,  are  borne  through  the  tubes  to  the  exterior. 

46.  Nervous  system  and  sense-organs. — In  the  sponges  no 
definite  nervous  system  is  known  to  exist,  the  slight  move- 
ments which  the  cells  are  able  to  undergo  being  regulated 
somewhat  as  they  are  in  the  Protozoa.     Among  the  coelen- 
terates certain  of  the  cells  scattered  over  the  surface  of  the 
body  are  set  aside  as  nerve-cells,  and,  more  or  less  united  by 
means  of  fibers  extending  from  them,  convey  impulses  over 
the  body.     In  the  flatworms  the  larger  number  of  nerve-cells 

26 


FIG.  29.— Flame-cell  of  flat- 
worm  (after  LANG).  /, 
flagellum  ;  n,  nucleus; 
s,  spaces  among  the  or- 
gans of  the  body  ;  v, 
waste  materials. 


48  ANIMAL  FORMS 

are  collected  into  two  definite  masses  (Fig.  28,  B),  which 
constitute  a  simple  brain  on  which  the  eyes  are  situated 
and  from  which  bundles  of  nerve  fibers  pass  to  all  parts  of 
the  body,  the  two  extending  backward  being  especially 
noticeable.  As  in  the  squirrel,  these  are  distributed  to  the 
muscles  and  other  organs  to  regulate  their  activity,  while 
those  distributed  to  the  skin,  especially  in  the  forward 
part  of  the  body,  convey  stimuli  produced  by  touch.  The 
branches  connecting  with  the  eyes  enable  the  animal  to 
distinguish  light  from  darkness,  but  are  probably  too  sim- 
ple to  allow  it  to  clearly  distinguish  objects  of  the  outside 
world.  The  sense  of  smell  and  possibly  that  of  taste  are 
also  present,  but  are  relatively  feeble. 

Some  other  characters  of  this  class  will  be  noted  in  the 
consideration  of  the  two  following  classes. 

47.  Parasitic  flatworms  (trematodes)—  parasitism.— Men- 
tion has  already  been  made  of  the  associations  of  two  ani- 
mals as  "  messmates "  for  mutual  benefit,  such  as  the  Hy- 
dractinia  growing  on  the  surface  of  the  shell  inhabited  by 
the  hermit  crab,  to  which  it  gives  protection  by  means  of 
its  nettle-cells,  while  in  turn  being  borne  continually  into 
regions  abounding  with  food.  More  frequently,  however, 
one  animal  derives  benefit  from  another  without  making 
any  compensation.  For  example,  many  species  of  flatworms 
live  within  the  shells  of  certain  snails  and  upon  the  bodies 
of  sea-urchins  and  starfishes,  where  they  gather  in  their 
food  supply  safe  from  the  attacks  of  enemies.  Such  asso- 
ciations are  probably  without  much  if  any  inconvenience  to 
the  animal  thus  inhabited,  and  it  also  appears  probable 
that  the  tenants  are  transients,  using  the  mollusk  or  star- 
fish only  as  a  temporary  home.  But  from  this  condition  of 
affairs  it  is  only  a  short  step  to  the  parasitic  habit,  where 
the  association  becomes  permanent  and  the  occupant  is 
provided  with  various  structures  which  prevent  its  sepa- 
ration from  its  host.  This  latter  kind  of  union  occurs 
throughout  the  group  of  trematodes ;  all  are  parasitic,  and 


THE   WORMS 


49 


their  internal  organization,  so  closely  resembling  that  of 
the  free-living  forms  as  to  need  no  further  description,  in- 
dicates that  they  are 
descendants  of  the  lat- 
ter. In  the  greater 
number  the  body  is 
flat,  and  a  few  species 
still  retain  their  outer 
coat  of  cilia  ;  but  since 
these  are  no  longer  of 
service  as  locomotor 
organs  they  have  gen- 
erally disappeared,  and 
in  their  place  numer- 
ous adhesive  organs, 
such  as  spines,  hooks, 
and  suckers  (Fig.  30), 
have  arisen,  which  en- 
able the  animals  to 
hold  on  with  great  te- 
nacity. Thus  attached 
to  its  host,  and  using 
it  as  a  convenient  and 
comparatively  safe 
means  of  locomotion, 
the  parasite  may  still 

continue  to  capture  small  animals  for  food  or  may  derive 
its  nourishment  from  the  tissues  of  the  host.  In  addition 
there  are  numbers  of  internal  parasites,  living  almost  ex- 
clusively in  the  bodies  of  vertebrate  animals,  scarcely  a  sin- 
gle one  escaping  their  ravages. 

48.  Life  history. — In  the  external  parasites  the  young 
hatch  out  and  with  comparative  ease  make  their  way  to 
another  host ;  but  the  young  of  an  internal  parasite,  inhab- 
iting the  alimentary  canal,  have  a  very  slight  chance  in- 
deed of  ever  reaching  a  similar  location  in  another  host. 


FIG.  30.  —  A  parasitic  flatwonn  (Epidella).  m 
month  ;  o,  opening  of  reproductive  system  ; 
s,  sucker  and  spines  for  attachment.  The  di- 
gestive system  is  stippled  ;  nervous  system 
black.  Enlarged  8  times,  from  Nature. 


50  ANIMAL  FORMS 

For  this  reason  an  almost  incredible  number  of  eggs  is  laid, 
and  some  extraordinary  measures  are  employed  in  effecting 
the  desired  result.  Probably  the  best-known  example  is  that 
of  the  liver  fluke  inhabiting  the  bile-ducts  in  the  sheep. 
Each  worm  lays  several  hundred  thousand  eggs,  which  make 
their  way  from  the  host,  and  if  they  chance  to  fall  "in  pools 
of  water  or  damp  situations  may  proceed  to  develop,  other- 
wise not.  If  the  surroundings  be  favorable,  the  young,  like 
little  ciliated  Infusoria,  escape  from  their  shells  and  rest- 
lessly swim  or  move  about  for  a  short  time,  and  if  during 
this  time  they  come  in  contact  with  certain  species  of 
snails  living  in  these  situations  they  at  once  bore  into  their 
bodies.  Here  they  produce  other  young  somewhat  resem- 
bling a  tadpole,  that  now  make  their  escape  from  the  snail. 
In  a  short  time  each  one  crawls  upon  a  blade  of  grass,  and 
surrounds  itself  with  a  tough  shell,  where  it  may  remain  for 
several  weeks.  If  the  grass  on  which  they  rest  be  eaten  by 
a  sheep,  they  finally  make  their  way  to  the  bile-ducts  and 
there  become  adult.  The  life  cycle  is  now  complete  ;  the 
young  form  has  found  a  new  host ;  and  the  process  shows 
how  wonderfully  animals  are  adapted  to  the  conditions  which 
surround  them,  and  how  closely  they  must  conform  to  these 
conditions  in  order  to  exist. 

49.  The  tapeworms  (cestodes), — The  cestodes,  or  tape- 
worms, are  also  parasitic  flatworms  in  which  the  effects  of 
such  a  mode  of  life  are  strongly  marked.  They  occur 
almost  exclusively  in  the  bodies  of  vortebrate  hosts  and 
exhibit  a  great  variety  of  bodily  forms,  in  some  cases  resem- 
bling rather  closely  the  trematodes,  but  in  others  strikingly 
different.  In  the  latter  type  the  body  is  usually  of  great 
length  (from  a  few  centimeters  to  upwards  of  sixteen  meters 
(50  feet)),  and  terminates  in  a  "head"  (Fig.  31)  provided, 
in  the  different  species,  with  a  great  variety  of  hooks  and 
spines  and  numbers  of  suckers  for  its  attachment  to  the 
body  of  the  host.  From  the  head  the  body  extends  back- 
ward in  the  gradually  enlarging  ribbon-like  body,  slender  at 


THE  WORMS 


51 


first  and  scarcely  showing  the  segments  which  finally  be- 
come so  prominent  a  feature. 

When  carefully  examined,  a  two-lohed  brain  is  found 
in  the  "  head,"  and  from  it  nerves  extend  the  entire  length 
of  the  body,  followed  throughout  their 
course  by  the  tubes  of  the  excretory 
system ;  also  each  segment  contains  a 
perfect  reproductive  system,  so  that 
even  if  it  be  separated  from  the  others 
it  may  continue  to  exist  for  a  consid- 
erable length  of  time.  Furthermore, 
the  tapeworms  are  surrounded  by  the 
predigested  fluids  of  their  host;  a 
special  alimentary  canal  is  therefore 
superfluous,  and  all  traces  of  it  have 
disappeared. 

50.  Development— As  the  animal 
clings  in  this  passive  way  to  the  body 
of  its  host  the  segments,  loaded  with 
eggs  ready  for  development,  separate 
one  after  another  from  the  free  end 
of  the  body,  pass  to  the  exterior,  and 

slowly  crawling  about  like  independent  organisms,  lay  great 
numbers  of  eggs,  which  may  find  an  intermediate  host  as  in 
the  life  cycle  of  the  liver  fluke,  and  so  in  time  find  their 
permanent  resting-place.  Fortunately  in  all  these  parasitic 
forms,  though  an  inconceivably  great  number  of  eggs  are 
laid,  only  a  comparatively  few  reach  maturity.  Even  these, 
however,  may  cause  at  times  great  destruction  among  the 
higher,  and  especially  our  domestic,  animals,  often  doing 
damage  amounting  to  many  millions  of  dollars  per  year. 

51.  The  tapeworm  in  relation  to  regeneration,— It  has 
been  known  for  more  than  one  hundred  and  fifty  years  that 
some  of  the  lower  animals  possess  to  a  surprising  degree 
the  ability  to  regenerate  parts  of  the  body  lost  through 
injury.     The  Hydra,  hydroids,  and  some  of  the  jelly-fishes 


FIG.  31.— Tapeworm  (Tcenia 
solium).  In  upper  left- 
hand  corner  of  figure  is 
the  much  enlarged  head. 
—After  LEUCKART. 


52  ANIMAL  FORMS 

may  be  cut  into  a  number  of  pieces,  each  of  which  will 
develop  into  a  complete  individual ;  and  this  power  of  recov- 
ery from  the  injuries  produced  by  enemies  is  of  the  great- 
est service  in  the  perpetuation  of  the  species.  This  ability 
is  also  present  in  certain  flatworms,  and  some  species  are 
known  which  voluntarily  separate  the  body  into  two  por- 
tions, each  of  which  becomes  an  adult.  In  other  species  a 
similar  process  results  in  the  formation  of  a  chain  of  six 
individuals,  placed  end  to  end,  the  chain  finally  breaking 
up  into  as  many  complete  worms.  It  is  possible  that  the 
tapeworm  may  also  be  looked  upon  as  a  great  chain  of 
united  individuals  produced  by  the  division  of  a  single 
original  parent,  which  becomes  adapted  for  attaching  the 
others  until  they  separate.  These  latter  are  capable  only  of 
a  very  sluggish  movement,  and,  devoid  of  mouth  and  ali- 
mentary canal,  are  not  able  to  digest  their  food,  but  their 
life  work  is  to  so  lay  their  eggs  that  they  may  develop  into 
other  individuals,  and  for  this  they  are  well  adapted. 

NEMATODES  (THREADWORMS) 

52.  General  characters.— This  class  of  worms  is  com- 
posed of  an  enormous  number  of  different  species,  some  para- 
sitic, others  free  all  or  a  portion  of  their  lives,  and  in  view  of 
the  fact  that  they  inhabit  the  most  diverse  situations  it  is 
remarkable  that  they  are  so  uniform  in  their  structure.  In 
all  the  body  is  slender,  and  the  general  features  of  its  organ- 
ization may  be  readily  understood  from  an  examination  of 
the  "  vinegar  eel  "  (Fig.  32,  A).  This  small  worm  (not  an 
eel),  a  millimeter  or  two  in  length,  lives  on  the  various  forms 
of  mold  that  grow  in  fermenting  fruit  juices,  especially 
after  a  little  sugar  or  paste  has  been  added.  A  tough  cuti- 
cle surrounds  the  body,  preserving  its  shape  and  at  the 
same  time  protecting  the  delicate  organs  against  the  action 
of  the  acids  in  which  it  lives.  Through  this  may  be  seen 
great  bands  of  muscles  extending  the  entire  length  of  the 
body  and  producing  the  wriggling  movements  of  swimming 


THE  WORMS 


53 


or  crawling.  They  also  give  support  to  a  brain,  which  is  in 
the  form  of  a  collar  encircling  the  pharynx  near  the  head, 
and  to  the  great  nerves  which  extend  from  it.  Still  fur- 
ther within  the  transparent  body  the  alimentary  canal  may 
be  distinguished  as  a  straight  tube 
passing  directly  through  the  ani- 
mal. The  alimentary  canal  lies 
freely  in  a  great  space,  the  body 
cavity,  traces  of  which  may  exist 
in  the  flatworms  in  the  form  of 
hollow  spaces  into  which  the 
kidneys  open.  It  is  possible  that 
in  this  form  also  the  kidneys  open 
into  this  space,  and  it  is  roomy 
enough  besides  to  afford  lodg- 
ment for  the  reproductive  organs 
in  addition  to  a  large  amount  of 
fluid  which  is  probably  somewhat 
of  the  nature  of  blood.  A  space  in 
some  respects  similar  to  this  occurs 
in  all  the  animals  above  this  group, 
and  as  we  shall  see,  it  is  often  cu- 
riously modified  and  serves  for  a 
number  of  different  and  highly  im- 
portant purposes.  In  the  round- 
worms  the  fluid  it  contains  proba- 
bly acts  in  the  nature  of  a  blood 
system,  distributing  the  food  and 
oxygen  to  various  parts  of  the  body  and  carrying  the  wastes 
to  the  kidneys  for  removal. 

53.  Multiplication. — In  the  matter  of  the  production  of 
new  individuals  the  greatest  differences  exist.  In  some 
threadworms,  for  example  the  "  vinegar  eel,"  eggs  develop 
within  the  body  and  the  young  are  born  with  the  form  of  the 
parent.  In  other  cases  the  eggs  are  laid  in  the  water,  where 
they,  too,  may  directly  grow  to  the  adult  condition ;  but  in 


FIG.  32.  —  Thread-  or  round- 
worms.  A,  vinegar  eel  ( An- 
guittuld) ;  m,  month  ;  ph., 
pharynx  ;  i,  intestine  ;  ov.. 
developing  young.  B,  Tri- 
china. From  Nature,  greatly 
enlarged. 


54  ANIMAL  FORMS 

the  greater  number  of  species  the  development  is  round- 
about, and  one  or  more  hosts  are  inhabited  before  the  young 
assume  the  adult  condition.  Such  is  the  case  with  the 
dreaded  Trichina  (Fig.  32,  B),  which  infests  the  bodies  of 
several  animals,  particularly  the  rat.  When  these  forms 
are  introduced  into  the  alimentary  canal  of  the  rat,  for 
example,  they  soon  lay  a  vast  quantity  of  eggs,  sometimes 
many  millions,  which  develop  into  young  that  bore  their 
way  into  the  muscles  of  the  body,  where  they  may  remain 
coiled  up  for  years.  If  the  body  of  the  rat  be  eaten  by  some 
carnivorous  animal,  these  excessively  small  young  are  lib- 
erated during  the  process  of  digestion  and  rapidly  assume 
the  adult  condition  in  the  alimentary  canal,  likewise  giving 
rise  to  young  which  pursue  again  this  same  course  of 
development. 

Another  example  of  a  complicated  life  history  is  in 
the  Gordius  or  "  horsehair  snake  "  (a  true  worm  and  not  a 
snake)  frequently  seen  in  the  spring  in  pools  where  it  lays 
its  eggs.  These  eggs  develop  into  young  which  bore  their 
way  into  different  insect  larvae,  which  are  in  turn  eaten  by 
some  spider  or  beetle,  and  the  worm  thus  transferred  to  a 
new  host.  In  this  they  grow  to  a  considerable  size,  and 
then  make  their  exit  from  the  body  of  the  host  and  finally 
become  adult. 

54.  Spontaneous  generation. — It  is  only  within  compara- 
tively recent  years  that  such  life  histories  have  been  under- 
stood. Formerly  the  sudden  appearance  of  these  and  other 
forms  in  various  situations  were  accounted  for  on  the  ground 
that  they  arose  spontaneously  without  the  intervention  of 
any  living  creature.  Even  yet  we  hear  of  the  transforma- 
tion of  horsehairs  into  hairworms,  and  of  frogs,  earthworms, 
and  several  other  animals  from  inorganic  matter,  but  such 
assertions  are  based  on  superficial  observations,  and  at  the 
present  time  no  exception  is  known  to  the  law  that  living 
creatures  arise  from  preexisting  living  parents.  "  All  life 
from  life  "  (omnium  vivum  ex  vivo)  is  a  universal  law. 


THE  WORMS  55 

ANNELIDS  OR  SEGMENTED  WORMS 

55.  The  earthworms  and  their  relatives.— Leaving  the 
groups  of  the  parasitic  animals,  which  have  been  driven  from 
the  field  of  active  existence  and  in  many  ways  are  degraded 
by  such  a  mode  of  life,  we  pass  on  to  the  higher  free-living 
worms,  where  brilliant  colors,  peculiar  habits,  or  remarkable 
adaptations  render  them  peculiarly  interesting.  In  consid- 
ering first  their  general  organization,  we  may  use  the  earth- 
C 


m 

FIG.  33.— Earthworm  (Lumbricus  terrestris).    m,  mouth  ;  c,  girdle  or  clitellum. 

worm  (Fig.  33)  (sometimes  called  angle-worm  or  fish-worm) 
as  a  type  because  of  its  almost  universal  distribution. 

The  body  is  cylindrical,  shows  well-marked  dorsal  and 
ventral  surfaces,  and,  as  in  all  of  the  annelids,  is  jointed, 
each  joint  being  known  as  a  segment.  Anteriorly  it  tapers 
to  a  point,  and  the  head  region  bearing  the  mouth  is  ill- 
defined,  unlike  many  sea  forms,  yet  serves  admirably  for 
tunneling  the  soil  in  which  all  earthworms  live.  In  this 
process  the  animal  is  also  aided  by  bristles  or  setce  which 
project  from  the  body  wall  of  almost  every  segment  and 
may  be  stuck  into  the  earth  to  afford  a  foothold. 

56.  Food  and  digestive  system. — The  earthworms  are 
nocturnal  animals,  seldom  coming  to  the  surface  during  the 
day  except  when  forced  to  do  so  by  the  filling  of  their  tun- 
nels with  water  or  when  pursued  by  enemies.  At  night 
they  usually  emerge  partially,  keeping  the  posterior  end  of 
the  body  within  the  burrow,  and  thus  they  scour  the  sur- 
rounding areas  for  food,  which  they  appear,  in  some  cases 
at  least,  to  locate  by  a  feeble  sense  of  smell.  They  also 
frequently  extend  their  habitations,  and  in  so  doing  swallow 
enormous  quantities  of  earth  from  which  they  digest  out 
any  nutritive  substances,  leaving  the  indigestible  matter  in 


56  ANIMAL  FORMS 

coiled  "  castings  "  at  the  entrance  of  the  burrows.  In  thus 
mixing  the  soil  and  rendering  it  porous  they  are  of  great 
service  to  the  agriculturist. 

Although  earthworms  are  omnivorous  they  also  manifest 
a  preference  for  certain  kinds  of  food,  notably  cabbage, 
celery,  and  meat,  which  leads  us  to  think  that  they  have  a 
sense  of  taste.  All  these  substances  are  carried  into  their 
retreats  and  devoured,  or  are  used  to  block  the  entrance 
during  the  day.  The  food  thus  carried  into  the  body  is 
digested  by  a  system  (Fig.  34)  composed  of  several  portions, 


FIG.  34. — Earthworm  (Lumbricus)  dissected  from  left  side,  b,  brain ;  c,  crop ; 
outer  opening  of  male  reproductive  system  ;  dv,  dorsal  blood-vessel ;  g,  gizzard  ; 
h,  pulsating  vessels  or  "  hearts  "  ;  i,  intestine  ;  k,  kidney  ;  m,  mouth  ;  n.  c.,  nerve- 
cord  ;  oe,  esophagus  ;  0,  ovary  ;  od,  oviduct ;  ph,  pharynx  ;  r,  testes ;  s.r.,  sem- 
inal receptacles  ;  v.v.,  ventral  vessel. 

each  of  which  is  modified  for  a  particular  part  in  the  pro- 
cess. The  mouth  (m)  leads  into  a  muscular  pharynx  (ph) 
whose  action  enables  the  worm  to  retain  its  hold  on  various 
objects  until  swallowed,  and  this  in  turn*  is  continuous  with 
the  esophagus.  From  here  the  food  is  passed  into  the  thin- 
walled  crop  (c),and  from  this  storehouse  is  gradually  borne 
into  the  gizzard  (^),  whose  muscular  walls  reduce  it  to  a  fine 
pulp  now  readily  acted  upon  by  the  digestive  fluids.  These, 
resembling  in  their  action  the  pancreatic  juice  of  higher 
animals,  are  poured  out  from  the  walls  of  the  intestine  into 
which  the  food  now  makes  its  way ;  and  as  it  courses  down 
this  relatively  simple  tube  the  nutritive  substances  are  ab- 
sorbed while  the  indigestible  matters  are  cast  away. 

57.  Circulatory  system. — In  all  the  groups  of  animals  up 
to  this  point  the  digested  food  is  carried  through  the  body 
by  a  simple  process  of  absorption,  or  in  the  threadworms  by 


THE  WORMS 


57 


means  of  the  fluid  in  the  body  cavity ;  but  in  the  earthworm 
the  division  of  labor  between  different  parts  of  the  body  is 
more  perfect,  and  a  definite  blood  system  now  acts  as  a 
distributing  apparatus.  This  consists  primarily  of  a  dorsal 
vessel  lying  along  the  dorsal  surface  of  tfee  alimentary  canal 
(Fig.  34),  from  which  numerous  branches  are  given  off  to 
thq  body  wall,  and  to  the  digestive  system  through  which 
they  ramify  in  every  direction  before  again  being  collected 
into  a  ventral  vessel  lying  below  the  digestive  tract.  In 
some  of  the  anterior  segments  a  few  of  the  connecting 
vessels  are  muscular  and  unbranched,  and  during  life  pul- 
sate like  so  many  hearts  to  force  the  blood  over  the  body, 
forward  in  the  dorsal  vessel,  through  the  "  hearts  "  into  the 
ventral  vessel,  thence  into  the  dorsal  by 
means  of  the  small  connecting  branches. 
Some  of  the  duties  of  this  vascular 
system  are  also  shared  by  the  fluid  of 
the  body  cavity,  which  is  made  to  cir- 
culate through  openings  in  the  parti- 
tions by  the  contractions  of  the  body 
wall  of  the  animal  in  the  act  of  crawl- 
ing. In  this  rough  fashion  a  consider- 
able amount  of  nutritive  material  and 
oxygen  are  distributed  to  various  or- 
gans, and  wastes  are  carried  to  the  kid-  FlG  35._Diagram  of  earth- 
neys  to  be  removed.  worm  kidney.  &,biood- 

T  i        11      £   J.T-  vessel ;  /,  funnel  open- 

58.  Excretion.— In  nearly  all  of  the        ing  into  body  cavity; 
segmented   worms   there   is   a   pair  of        o,  outer  opening;  *, 

j.  /TV         04    OK\  septum;  w,  body  wall. 

kidneys  to  every  segment  (Figs.  d4,  65). 
Each  consists  of  a  coiled  tube  wrapped  in  a  mass  of  small 
blood-vessels,  and  at  its  inner  end  communicating  with  the 
body  cavity  by  means  of  a  funnel-shaped  opening.  In 
some  unknown  way  jhe  walls  of  the  kidney  extract  the 
waste  materials  from  the  blood-vessels  coursing  over  it  and 
pass  them  into  its  tubular  cavity.  At  the  same  time  the 
cilia  about  the  mouth  of  the  funnel-shaped  extremity  are 


58  ANIMAL   FORMS 

driving  a  current  from  the  body-cavity  fluids,  which  wash 
the  wastes  to  the  exterior. 

59.  Nervous  system. — The  nervous  system  of  the  earth- 
worm consists  first  of  a  brain  composed  of  two  pear-shaped 
masses  united  together  above  the  pharynx  (one  shown  in 
Fig.  34),  from  which  nerves  pass  out  to  the  upper  lip  and 
the  head,  which  are  thus  rendered  highly  sensitive.     Two 
other  nerves  also  pass  out  from  the  brain,  and,  coursing 
down  on  each  side  of  the  pharynx  like  a  collar,  unite  below 
it  and  extend  side  by  side  along  the  under  surface  of  the 
digestive   system  throughout  its  entire  extent.     In  each 
segment  the  two  halves  of  this  ventral  nerve-cord  are  united 
by  a  nerve,  and -others  are  distributed  to  Various  organs, 
which  are  thus  made  to  act  and  in  proper  amount  for  the 
good  of  the  body  as  a  whole. 

In  its  relation  to  the  outside  world  the  chief  source  of 
information  comes  to  the  earthworm  through  the  sense  of 
touch,  for  definite  organs  of  sight,  taste,  and  smell  are  but 
feebly  developed,  while  ears  appear  to  be  entirely  absent. 
Nevertheless  these  are  sufficient  to  enable  it  to  lead  a  suc- 
cessful life,  as  is  evidenced  by  the  great  number  of  such 
worms  found  on  every  hand. 

60.  Egg-laying.— In  digging  up  the  soil  where   earth- 
worms  abound   one   frequently   finds   small   yellowish    or 
brownish  bodies  looking  something  like  a  gi*aiij  of  wheat. 
These  are  the  cocoons  in  which  the  earthworms  lay  their 
eggs,  and  the  method  by  which  this  is  performed  is  unique. 
We  have  already  noted  the  presence  of  a  swollen  girdle 
(the  clitellum)  about  the  body  of  the  worm.     At  the  breed- 
ing season  this  throws  out  a  fluid  which  soon  hardens  into 
an  encircling  band.     By  vigorous  contractions  of  the  body 
this  horn-like  collar  is  now  slipped  forward,  and  as  it  passes 
the   openings   of  the  reproductive   organs   the   eggs   and 
sperms  are  pushed  within  it.     They  thus  occupy  the  space 
between  the  worm  and  the  collar,  and  when  the  latter  is 
shoved  off  over  the  head  its  ends  close  as  though  drawn  to- 


THE   WORMS 


59 


gether  by  elastic  bands.     A  sac,  the  cocoon,  is  thus  pro- 
duced, containing  the  eggs  and  a  milky,  nutritive  substance. 
In  a  few  weeks  the  worm 
develops  and,  bursting  the 
wall  of  its  prison,  makes  its 
escape. 

61.  Distribution.  —  The 
earthworms  and  their  allies 
are  found  widely  distributed 
throughout  the  world,  and 
all    exhibit    many    of    the 
characters    just    described. 
The     greatest     differences 
arise  in  their  mode  of  life  : 
some  are  truly  earthworms, 
but  others  are  fitted  for  a 
purely  aquatic  existence  in 
fresh    water   or   along    the 
seacoast ;  a  few  have  taken 
up   abodes   in    various  ani- 
mals   and    plants,   and    in 
some  of  these  situations  they 
extend  far  up  the  sides  of 
the  higher  mountains.     In 
all,   the   head   is   relatively 
indistinct,    the    number   of 

bristles     011     each      Segment    FlG-  36.— A  marine  worm  (Nereis).    A,  ap- 

few,  and  for  this  and  other        ES^*""1*  8eason'  and  B' 

reasons  all  are  included  in 

the  subclass  Oligochaeta,  or  "  few-bristle  "  worms.  , 

62.  Nereis  and  its  allies. — In  many  of  the  above-men- 
tioned situations  members  of  a  more  extensive  group  of 
worms  are  found,  with  highly  developed  heads  and  many 
bristles  arranged  along  the  sides  of  the  body.     These  are 
the  Polychaetes  or  "  many-bristle  "  worms,  and  .as  a  repre- 
sentative we  may  take  Nereis  (Fig.  36),  a  very  common 


60  ANIMAL  FORMS 

form  along  almost  any  seashore.     The  body  presents  the 
same   segmented   appearance  as  the   earthworm,  but  the 
head  (Fig.  37,  A)  is  provided  with  numerous  sense  organs, 
chief  among  which  are  four  eyes  and 
several  tentacles  or  "feelers." 

The   segments  behind  the   head 


FIG.  37.— A,  head  and  one  of  the  lateral  appendages  (B)  of  a  marine  worm  (Nereis 
brandtii) ;  al,  intestine  ;  /,  "  gill  "  ;  k,  kidney  ;  ra,  nerve  cord  ;  s,  bristles  for  loco- 
motion. 

differ  very  little  from  one  another,  and,  unlike  those  of 
the  earthworm,  each  bears  a  pair  of  lateral  plates  (Figs. 
36,  37,  B)  or  paddles  with  many  lobes,  some  of  which  bear 
numerous  bristles.  By  a  to-and-f ro  movement  these  organs 
aid  in  pushing  the  animal  about,  or  may  enable  certain  spe- 
cies to  swim  with  considerable  rapidity. 

As  in  all  otn^r  worms,  respiration  takes  place  through 
the  surface  of  the  body,  the  area  of  which  is  increased  by 
the  development,  on  certain  portions  of  the  paddles  (para- 
podia),  of  plates  penetrated  with  numerous  blood-vessels, 
which  thus  become  special  respiratory  organs  or  gills 
(Fig.  37,  B). 

In  their  internal  organization  the  Polychaetes  are  con- 
structed practically  on  the  same  plan  as  the  earthworms, 
the  principal  difference  being  in  the  reproductive  system. 
In  the  earthworm  this  is  restricted  to  some  of  the  forward 
segments,  while  in  the  present  group  the  eggs  and  sperms 


THE  WORMS 


61 


are  developed  in  almost  every  segment,  whence  they  are 
finally  swept  to  the  exterior  through  the  tubes  of  the  kid- 
neys (Fig.  37,  B). 

The  Nereis  and  its  immediate  relatives  are  all  active 
forms,  and  by  means  of  powerful  jaws,  which  may  be  quickly 
extended  from  the  lower  part  of  the  mouth  cavity,  they 
capture  large  numbers  of  small  crustaceans,  mollusks,  and 
worms  which  happen  in  their  path.  Others  more  distantly 
related  make  their  diet  of  seaweed,  and 
many  living  on  the  sea  bottom  swallow 
great  quantities  of  sand,  from  which  they 
absorb  the  nutritious  substances. 

63.  Sedentary  forms. — Preyed  upon  by 
many  enemies,  a  large  number  of  species 
have  been  forced  to  abandon  an  active  ex- 
istence save  in  their  early  youth,  and  to 
construct  many  interesting  devices  for  their 
protection.  Numerous  species,  shortly  after 
they  commence  to  shift  for  themselves) 
build  about  their  bodies  tubes  of  lime  (Fig. 
39),  from  which  they  may  emerge  to  gather 
food  and  into  which  they  may  dash  in  times 
of  danger.  As  the  worm  grows  the  tube  is 
correspondingly  enlarged,  and  these  tubes, 
in  all  stages  of  construction  and  variously 
coiled,  may  be  found  on  almost  every  avail- 
able spot  at  the  seashore,  and  may  often 
be  seen  on  the  shells  of  oysters  in  the 
markets. 

In  other  species  the  tube  is  like  thin 
horn,  and  may  be  further  strengthened  or 
concealed  by  numerous  pebbles,  bits  of  carefully  selected 
seaweeds,  or  highly  tinted  shells,  which  give  them  a  very 
attractive  appearance.  Such  species  usually  develop  out 
of  immediate  contact  with  other  forms,  but  a  few  live 
so  closely  associated  together  that  their  twisted  tubes 


FIG.  38.— A  common 
marine  worm  (Po- 
ly nee,  brevisetosa), 
with  extended  pro- 
boscis and  over- 
lapping plates  cov- 
ering the  back. 


62 


ANIMAL  FORMS 


form  great  stony  masses,  sometimes  several  feet  in  dia- 
meter. 

64.  Effects  of  an  inactive  life. — In  many  species  such  a 
sedentary  life  has  resulted  in  the  almost  complete  disap- 
pearance of  the  lateral  appendages,  which  therefore  no 
longer  serve  as  organs  of  respiration,  and  this  function  has 
been  shifted  accordingly  on  to  other  structures.  These 
new  organs  are  situated  principally  on  the  exposed  head, 


FIG.  39.— Sedentary  tube-dwelling  marine  worms,  upper  left  hand  Sabella  (one-half 
natural  size),  the  remainder  Serpula  (enlarged  twice).    From  life. 

and  Fig.  39  shows  the  general  appearance  of  some  com- 
mon species.  The  corners  of  the  mouth  have  expanded 
into  great  plumes,  sometimes  wondrously  colored  like  a 
full-blown  flower,  and  these,  bounteously  supplied  with 
blood-vessels,  act  as  gills.  When  disturbed,  the  plumes  are 
hastily  withdrawn  into  the  tube,  and  some  of  the  so-called 
serpulids  (Fig.  39,  bottom  of  figure)  close  the  entrance  with 
a  funnel-shaped  stopper.  While  the  plumes  are  primarily 
respiratory  organs,  they  also  act  as  delicate  feelers,  and  may 
even  bear  a  score  or  more  of  eyes ;  and  in  addition,  being 


THE  WORMS 


63 


covered  with  cilia,  create  the  currents  of  water  which 
bring  minute  organisms  serving  as  food  within  reach  of 
the  mouth. 

65.  Development.— Unlike    the   earthworms,  the  Poly- 
chaetes  lay  their  eggs  in  the  sea  water,  where  they  are  left 
alone  to  develop  as  best  they  may.     Both  the  male  and 
female  Nereis,  as  the  egg-laying  time  approaches,  undergo 
remarkable  changes  in  their  external  appearance,  resulting 
in   the   form   shown  in   Fig.   36,  A. 

They  are  now  active  swimmers,  and 
thus  are  able  to  scatter  the  fertilized 
eggs  over  wide  and  more  or  less  favor- 
able areas.  The  young  also  for  a 
time  are  free-swimming,  but  finally 
end  their  migrations  by  settling  to 
the  sea  bottom,  where  they  gradually 
attain  the  adult  condition. 

As  in  some  of  the  flatworms,  re- 
production may  also  occur  asexually 
by  the  division  of  the  animal  into  two 
or  more  parts,  each  of  which  subse- 
quently becomes  a  complete  indi- 
vidual. In  other  species  growth  of 
various  parts  may  result  in  two  com- 
plete worms  at  the  time  of  separation ; 
and  from  such  forms  we  may  trace  a 
fairly  complete  series  up  to  those  in 
which  the  original  parent  breaks  up  Fie.4o.-A 
into  twenty  to  thirty  young. 

66.  The    leeches.  —  At   first  sight 
the  leeches  (Fig.  40),  or  at  least  the 
smaller,  more  leaf-like  forms,  might 

be  mistaken  for  flatworms,  especially  for  some  of  the  para- 
sitic species.  As  in  the  latter,  the  mouth  is  surrounded  by 
a  sucker,  and  another  is  located  at  the  hinder  end  of  the 
body,  but  beyond  this  point  the  resemblance  ceases.  The 
27 


I 


P. 

si. 


ld).    Right-hand  figure  il- 
lustrates  alimentary  canal. 

ph,  pharynx ;  e,  crop ;  p, 
;^.pouche8:  '*"  "* 


64  ANIMAL  FORMS 

outer  surface  is  delicately  marked  off  into  eighty  or  a  hun- 
dred rings,  of  which  from  three  to  five  are  included  in  one 
of  the  deeper  true  segments  corresponding  to  those  of 
other  annelids.  From  two  to  ten  pairs  of  simple  eyes  are 
borne  on  the  head,  and  owing  to  the  fact  that  they  are 
active  swimmers,  or  move  by  caterpillar-like  looping,  loco- 
motor  spines  are  unnecessary  and  absent.  In  their  internal 
organization,  however,  there  are  many  features  which  in- 
dicate a  close  relationship  with  the  Oligochaetes  or  few- 
bristle  worms.  The  nervous,  circulatory,  and  certain  char- 
acteristics of  the  excretory  systems  are  decidedly  similar, 
but,  on  the  other  hand,  there  are  some  facts  difficult  to 
explain,  which  have  led  some  zoologists  to  believe  that 
the  relationship  of  these  animals  can  not  at  present  be 
determined. 

67.  Haunts  and  habits.— The  leeches  usually  dwell  in 
among  the  plants  in  slowly  running  streams,  but  some 
occur  in  moist  haunts  on  land,  and  a  considerable  number 
live  in  the  sea.  All  are  "  bloodsuckers  " — fierce  carnivo- 
rous worms,  whose  bite  is  so  insidiously  made  that  the  vic- 
tim frequently  is  ignorant  of  their  presence.  Fishes,  frogs, 
and  turtles  are  the  most  frequently  attacked,  but  cattle  and 
other  animals  which  come  down  to  drink  also  become  their 
prey.  In  some  of  the  tropical  countries  the  land-leeches 
are  present  in  large  numbers  secreted  among  the  leaves,  and 
so  severe  are  their  attacks  that  various  animals,  even  man, 
succumb  to  their  united  efforts.  Adhering  by  their  suck- 
ers, they  puncture  the  skin,  some  using  triple  jaws,  and 
fill  themselves  until  they  become  greatly  distended,  when 
they  usually  drop  off  and  digest  the  meal  at  leisure.  In 
certain  species  the  intestine  is  provided  with  lateral 
pouches  (Fig.  40),  which  serve  to  store  up  the  food  until 
the  time  for  digestion  arrives.  A  full  meal  is  sufficient 
with  some  species  to  last  for  two  or  three  months,  and  the 
medicinal  or  horse-leech  when  gorged  with  food  may  con- 
sume a  year  in  digesting  it. 


THE  WORMS  65 

68.  Egg-laying. — The  eggs  of  some  leeches  are  stored 
up  in  a  cocoon  like  that  of  the  earthworm,  which  is  attached 
to  submerged  plants  or  placed  under  stones.  When  the 
young  are  able  to  lead  independent  lives  they  emerge  with 
the  form  of  the  parent.  A  leaf -like  form,  Clepsine,  some- 
times found  adhering  to  turtles,  fastens  the  eggs  to  the 
under  side  of  its  body,  and  the  young  when  hatched 
remain  there  for  several  days,  adhering  by  their  posterior 
suckers. 


CHAPTER  VII 


ANIMALS   OF   UNCERTAIN   RELATIONSHIPS 

IN"  this  chapter  we  shall  consider  in  a  brief  way  a  number 
of  different  groups  of  animals  whose  relationships  are  un- 
certain. Up  to  the  present  time  the  study  of  their  habits, 
structure,  and  development  has  been  of  too  fragmentary 
or  unrelated  a  character  to  enable  the  majority  of  zoologists 
to  agree  upon  their  classification.  Nevertheless,  many  of 
them  are  highly  interesting  and  attractive, 
often  very  common,  and  in  some  respects 
they  hold  important  positions  in  the  animal 
kingdom. 

69.  The  rotifers  or  wheel-animalcules. — 
The  rotifers  or  wheel-animalcules  are  rela- 
tively small  and  beautiful  organisms,  rarely 
ever  longer  than  a  third  of  an  inch,  but  at 
times  so  abundant  that  they  may  impart  a 
reddish  tinge  to  the  water  of  the  streams 
and  ponds  in  which  they  live.  At  first 
sight  they  might  be  mistaken  for  one-celled 
animals,  but  the  presence  of  a  digestive 
tract  and  of  reproductive  elements  soon  dis- 
pels such  a  belief.  Examined  under  the 
microscope,  the  more  common  forms  are 
seen  to  possess  an  elongated  body  terminat- 
ing at  the  forward  end  in  two  disk-like  expansions  beset 
along  the  edges  with  powerful  cilia.  These  serve  to  drive 
the  animal  about,  or,  when  it  remains  temporarily  attached 


FIG.  41.— A  wheel- 
animalcule  (Rotifer). 


ANIMALS  OF  UNCERTAIN  RELATIONSHIPS  67 

by  the  sticky  secretion  of  the  foot,  to  sweep  the  food-par- 
ticles down  into  the  mouth.  Through  the  walls  of  the 
transparent  body  such  substances  are  seen  to  pass  into  the 
stomach,  where  they  are  rapidly  hammered  or  rasped  into 
a  pulp  by  the  action  of  several  teeth  located  there.  In 
the  absence  of  a  circulatory  system  the  absorbed  food  is 
conveyed  by  the  fluid  of  the  body-cavity,  which  also  con- 
veys the  wastes  to  the  delicate  kidneys.  Several  other 
features  of  their  organization  are  of  much  interest,  espe- 
cially to  the  zoologist,  who  believes  that  he  gains  from 
their  simple  structure  some  ideas  of  the  ancestors  of  the 
modern  worms,  mollusks,  and  their  allies.  During  the 
summer  the  rotifers  lay  two  sizes  of  "  summer  eggs," 
which  are  remarkable  for  developing  without  fertilization. 
The  large  size  give  rise  to  females,  the  smaller  to  males,  the 
latter  appearing  when  the  conditions  commence  to  be  un- 
favorable. The  "  winter  eggs,"  fertilized  by  the  males  and 
covered  with  a  firm  shell,  are  able  for  prolonged  periods  to 
withstand  freezing,  drought,  or  transportation  by  the  wind. 
The  adults  also  are  able  under  the  same  adverse  conditions 
to  surround  themselves  with  a  firm  protective  membrane 
and  to  exist  for  at  least  a  year.  Once  again  in  the  presence 
of  moisture  the  shell  dissolves,  and  in  a  surprisingly  short 
space  of  time  they  emerge,  apparently  none  the  worse  for 
the  prolonged  period  of  quiescence. 

70.  Gephyrea.— There  is  a  comparatively  large  group  of 
worm-like  organisms,  over  one  hundred  species  in  all,  which 
at  present  hold  a  rather  unsettled  position  in  the  animal 
kingdom.  Some  of  the  more  common  forms  (Fig.  42) 
living  in  the  cracks  of  rocks  or  buried  in  the  sand,  usually 
in  shallow  tide  pools  along  the  seashore,  have  a  spindle- 
shaped  body  terminated  at  one  end  by  a  circlet  of  tentacles 
which  surround  the  mouth.  On  account  of  their  external 
resemblance  to  many  of  the  sea-cucumbers  (Fig.  92),  they 
were  earlier  associated  in  the  same  group ;  but  an  examina- 
tion of  their  internal  organization  inclines  many  zoologists 


ANIMAL  FORMS 


to  the  belief  that  the  ancestors  of  some  of  these  animals 
were  segmented  worms  whose  present  condition  has  arisen 
possibly  in  accordance  with  their  sluggish  habits.  This 
view  is  strengthened  by  the  fact  that  in  a  very  few  species 

the  larvae  are  dis- 
tinctly segmented, 
but  lose  this  char- 
acter in  becoming 
adult.  As  before 
mentioned,  the 
greater  number  of 
species  live  in  bur- 
rows in  the  sand 
or  crevices  in  the 
rocks,  from  which 
they  reach  out  and 
gather  in  large 
quantities  of  sand. 
As  these  substances 
pass  down  the  in- 
testine the  nutri- 
tive matters  are  di- 
gested and  absorbed,  while  the  indigestible  matters  are 
voided  to  the  exterior.  When  large  numbers  are  associated 
together  they  are  doubtless  important  agents  in  modifying 
the  character  of  the  sea  bottom,  thus  acting  like  the  earth- 
worms and  their  relatives. 

71.  The  sea-mats  (Polyzoa), — The  sea-mats  or  Polyzoa 
constitute  a  very  extensive  group  of  animals  common  on 
the  rocks  and  plants  along  the  seashore,  and  frequently 
seen  in  similar  situations  in  fresh-water  streams.  A  few 
lead  lives  as  solitary  individuals,  but  in  the  greater  number 
of  species  the  original  single  animal  branches  many  times, 
giving  rise  to  extensive  colonies.  In  some  species  these 
extend  as  low  encrusting  sheets  over  the  objects  on  which 
they  rest;  while  in  others  the  branches  extend  into  the 


m~ 


FIG.  42.— A  gephyrean  worm  (Dendrostoma).  Specimen 
on  left  opened  to  show  £,  kidney,  m,  muscle  bands, 
and  n.c.,  nerve-cord. 


ANIMALS  OF  UNCERTAIN  RELATIONSHIPS 


69 


surrounding  medium  and  assume  feathery  shapes  (Fig.  43), 
which  often  bear  so  close  a  resemblance  to  certain  plants 


FIG.  43.-Lamp-ehells  or  Brachiopods  (on  left  of  figure),  fossil  and  living,  and  (on 
right)  plant-like  colonies  of  sea-mats. 

that  they  are  frequently  preserved  as  such.  What  their 
exact  position  is  in  the  animal  scale  it  is  somewhat  difficult 
to  say ;  but  judging  especially  from  their  development,  it 
appears  probable  that  they  are  distant  relatives  of  the  seg- 
mented worms. 


70  ANIMAL  FORMS 

72.  Lamp-shells  or  Brachiopods. — Occasionally  one  may 
find  cast  on   the  beach  or   entangled   in   the   fishermen's 
lines  or  nets  a  curious  bivalve  animal  similar  to  the  form 
shown  in  Fig.  43.      These  are  the  Brachiopods,  or  lamp- 
shells.     The   remains   of   closely  related   forms   are   often 
abundant  as  fossils  in  the  rocks  (Fig.  43).    Over  a  thousand 
species  have  been  preserved  in  this  way,  and  we  know  that 
in  ages  past  they  flourished  in  almost  incredible  numbers 
and  were  scattered  widely  over  the  earth.    Unable  to  adapt 
themselves  to  changing  conditions  or  unable  to  cope  with 
their  enemies,  they  have  gradually  become  extinct,  until 
to-day  scarcely  more  than  one  hundred  species  are  known. 
These  are  often  of  local  distribution,  and  many  are  com- 
paratively rare. 

For  a  long  period  the  Brachiopods,  owing  to  their  pecul- 
iar shells,  were  classed  together  with  the  clams  and  other 
bivalve  mollusks.  The  presence  of  a  mantle  also  strength- 
ened the  belief ;  but  closer  examination  during  more  recent 
years  has  shown  that  the  shells  are  dorsal  and  ventral,  and 
not  arranged  against  the  sides  of  the  animal  as  in  the 
clams.  Another  peculiar  structure  consists  of  two  great 
spirally  coiled  "  arms,"  which  are  comparable  in  a  general 
way  to  greatly  expanded  lips.  The  cilia  on  these  create,  in 
the  water  currents  which  sweep  into  the  mouth,  the  small 
animals  and  plants  that  serve  as  food.  The  internal  organ- 
ization resembles  in  a  broad  way  that  of  the  animals  con- 
sidered in  the  previous  section,  and  it  now  appears  that 
both  trace  their  ancestry  back  to  the  early  segmented 
worms. 

73.  Band  or  nemertean  worms. — In  a  few  cases  band  or 
nemertean  worms  have  been  discovered  in  damp  soil  or  in 
fresh-water  streams.    These  are  commonly  small  and  incon- 
spicuous, and  are  pigmies  when  compared  with  their  marine 
relatives,  which  sometimes  reach  a  length  of  from  fifty  to 
eighty  feet.     Many  of  the  marine  species  (Fig.  44)  are  often 
found  on  the  seashore  under  rocks  that  have  been  exposed 


ANIMALS  OF  UNCERTAIN  RELATIONSHIPS 


71 


B 


by  the  retreating  tide.  They  are  usually  highly  colored 
with  yellow,  green,  violet,  or  various  shades  of  red,  and  are 
so  twisted  into  tangled 
masses  that  the  differ- 
ent parts  of  the  body 
are  indistinguishable. 
As  the  animal  crawls 
about,  a  long  thread- 
like appendage,  the  pro- 
boscis, is  frequently  shot 
out  from  its  sheath  at 
the  forward  end  of  the 
body  and  appears  to  be 
used  as  a  blind  man 
uses  his  stick.  At  other 

times,  when  Small  WOrmS  spine-tipped  proboscis. 

and  other  animals  are 

encountered,  the  proboscis  is  shot  out  farther  and  with 
greater  force,  impaling  the  victim  on  a  sharp  terminal  spine 
(Fig.  44).  The  food  is  now  borne  to  the  mouth,  located 
near  the  base  of  the  proboscis,  is  passed  into  the  digestive 
tract,  traversing  the  entire  length  of  the  body,  and  is  far- 
ther operated  on  by  systems  of  organs  too  complex  to  be 
considered  here. 


FIG.  44.— A  band  or  nemertean  worm.    A,  entire 
worm ;  B,  head,  bearing  numerous  eyes  and 


CHAPTEE  VIII 

MOLLUSKS 

74.  General  characters. — For  very  many  years  the  mol- 
lusks — that  is,  the  clams,  snails,  cuttlefishes,  and  their  allies 
— have  been  favorite  objects  of  study  largely  because  of  the 
durability,  grace,  and  coloration  of  the  shell.     The  latter 
may  be  univalve,  consisting  of  one  piece,  as  in  the  snails,  or 
bivalve,  as  in  the  clams  and  mussels,  and  may  possess  almost 
every  conceivable  shape,  and  vary  in  size  from  a  grain  of 
rice  to  those  of  the  giant  clam  (Tridacna)  of  the  East  Indian 
seas,  which  sometimes  weighs  five  hundred  pounds.    These 
external  differences  are  but  the  expression  of  many  internal 
modifications,  which,  while  adapting  these  animals  for  dif- 
ferent modes  of  life,  are  yet  not  sufficient  to  disguise  a 
more  fundamental  resemblance  which  exists   throughout 
the  group.     In  some  respects  the  mollusks  show  a  close 
resemblance  to  the  annelid  worms,  but,  on  the  other  hand, 
the  body  is  usually  more  thick-set  and  totally  devoid  of  any 
signs  of  segmentation.     In  every  case  the  skin  is  soft  and 
slimy,  demanding  moist  haunts  and  usually  the  protection 
of  a  shell,  and  the  body  is  modified  along  one  surface  to 
form  a  foot  or  creeping  disk  which  serves  in  locomotion. 
The  internal  organization  is  somewhat  uniform,,  and  will 
admit   of   a   general   description   later   on.     Mollusks   are 
divided  into  three  classes,  viz. :  The  Lamellibranchs,  em- 
bracing the  clams ;   the  Gasteropods,  or  snails ;  and  the 
Cephalopods,  or  cuttlefishes,  squids,  and  related  forms. 

75.  Lamellibranchs  (clams  and  mussels). — Numerous  rep- 
resentatives of  this  class,  such  as  the  clams  and  mussels, 

72 


MOLLUSKS  73 

occur  along  our  seacoasts  or  are  plentifully  distributed  in 
the  fresh-water  streams  and  lakes.  >  They  are  distinguished 
from  other  mollusks  by  a  greatly  compressed  body,  which 
is  enclosed  in  a  shell  consisting  of  two  pieces  or  valves 
locked  together  by  a  hinge  along  the  dorsal  surface.  Eais- 
ing  one  of  these  valves,  the  main  part  of  the  body  may  be 
seen  to  occupy  almost  completely  the  upper  (dorsal)  part 
of  the  shell  (Fig.  45),  and  to  be  continued  below  into  the 
muscular  hatchet-shaped  foot  (/£.),  which  aids  the  clam  in 
plowing  its  way  through  the  sand  or  mud  in  which  it  lives. 
Arising  on  each  side  of  the  back  of  the  animal  and  extend- 
ing its  entire  length  is  a  great  fold  of  skin,  which  com- 
pletely lines  the  inner  surface  of  the  corresponding  valve 
of  the  shell.  These  are  the  two  mantle  lobes  (m)  instru- 
mental in  the  formation  of  the  shell,  and  enclosing  between 
them  a  space  containing  the  foot  and  a  number  of  other 
important  structures,  the  most  conspicuous  of  which  are 
the  gills  (</),  consisting  of  two  broad,  thin  plates  attached 
along  the  sides  of  the  animal  and  hanging  freely  into  the 
space  (mantle  cavity)  between  the  mantle  and  the  foot. 
Owing  to  this  lamella-like  character  of  the  branchia  or  gills 
the  class  derives  its  name,  lamellibranch.  To  illustrate  the 
relations  of  these  various  organs  to  one  another  the  clam  has 
been  compared  to  a  book,  in  which  the  shells  are  repre- 
sented by  the  cover,  the  fly-leaves  by  the  mantle  lobes,  the 
first  two  and  last  two  pages  by  the  gills,  and  the  remaining 
leaves  by  the  foot.  In  the  clams,  however,  the  halves  of 
the  mantle,  like  the  halves  of  the  shell,  are  curved,  and 
thus  enclose  a  space,  the  mantle  cavity,  which  is  partly 
filled  by  the  gills  and  foot. 

Unlike  the  other  mollusks  which  usually  lead  active 
and  more  aggressive  lives,  the  clams  show  scarcely  a  sign  of 
a  head  and  tentacles,  and  other  sense  organs  are  likewise 
absent  from  this  region.  The  mouth  also  lacks  definite 
organs  of  mastication,  and  as  devices  for  catching  and 
holding  food  are  not  developed,  the  food  is  brought  to  the 


74: 


ANIMAL  FORMS 


mouth  by  means  of  the  cilia  on  the  great  triangular  lips  or 
palps  which  bound  it  on  each  side  (Fig.  45,  A,^?). 

h 


FIG.  45. — Anatomy  of  fresh-water  clam.  A,  right  valve  of  shell  removed  ;  B,  dissec 
tion  to  show  internal  organs,  a,  external  opening  of  kidney ;  a.a.,  the  anterior 
muscle  for  closing  the  shell ;  5,  opening  of  reproductive  kidney  ;  c,  brain  ;  ft., 
foot ;  g,  gill ;  h,  heart ;  i,  intestine  ;  k,  kidney  ;  I,  liver  ;  m,  mantle  (upper  fig.), 
mouth  (lower  fig.) ;  p,  palp  (upper  fig.),  foot  nerves  (lower  fig.) ;  p.a.,  hinder 
muscle  for  closing  the  shell  ;  s,  space  through  which  the  water  passes  on 
leaving  the  body  ,  "  t,  stomach  ;  v,  nerves  supplying^viscera. 

Between  the  halves  of  the  shell  in  the  hinge  region  is  a 
horny  pad  that  acts  like  a  spring,  and  without  any  muscu- 
lar effort  on  the  part  of  the  clam  keeps  the  shells  open. 


MOLL  USES  75 

These  are  also  united  by  two  great  adductor  muscles,  located 
at  opposite  ends  of  the  animal  (Fig.  45,  A,  «.«.,  p.a.),  which 
in  times  of  disturbance  contract  and  firmly  close  the  shell. 
Upon  their  relaxation  the  shell  opens,  the  clam  extends  its 
foot,  and  plows  its  way  leisurely  through  the  mud,  or  re- 
mains buried,  leaving  only  the  hinder  portion  of  its  gaping 
shell  exposed.  Through  this  opening  a  current  of  water 
is  continually  passing  in  and  out,  owing  to  the  action  of 
the  cilia  covering  the  gills,  and  by  placing  a  little  car- 
mine or  coloring  matter  in  the  ingoing  stream  we  may 
trace  its  course  through  the  body.  Passing  in  between  the 
mantle  and  the  foot  it  travels  on  toward  the  head,  giving 
off  small  side  streams  which  are  continually  made  to  enter 
minute  openings  in  the  gills,  whence  they  are  conducted 
through  tubes  in  each  gill  up  to  a  large  canal  at  its  base, 
where  it  is  carried  backward  to  the  exterior.  In  this  pro- 
cess oxygen  gas  is  supplied  to  the  number  of  blood-ves- 
sels traversing  the  gills,  and  at  the  same  time  considerable 
quantities  of  minute  organisms  and  organic  debris  are 
hurried  forward  toward  the  head,  where  they  encounter  the 
whirlpools  made  by  the  cilia  on  the  lips  and  are  rapidly 
whisked  down  into  the  mouth  and  swallowed. 

75.  Rock-  and  wood-boring  clams. — Other  similar  forms 
are  rendered  even  more  secure  through  their  ability  to 
bore  in  solid  rock.  In  the  common  Piddock,  for  example 
(Fig.  46),  the  shell  is  beset  with  teeth  like  a  rasp,  which 
gradually  enlarge  the  cavity  as  the  animal  grows,  until  it 
becomes  a  prisoner  with  no  means  of  communication  with 
the  exterior  save  the  small  opening  through  which  the 
siphons  project.  This  is  also  the  case  with  the  Teredo, 
frequently  called  the  shipworm,  which  swims  about  for 
some  time  during  early  life  and  then,  about  the  size  of  a 
small  pinhead,  settles  down  upon  the  timbers  of  wharves 
or  unsheathed  ships,  into  which  it  rapidly  tunnels. 
Throughout  life  its  excavation  is  extended  sometimes  to  a 
distance  of  two  to  three  feet,  and  imprisoned  yet  safe  at 


76  ANIMAL  FORMS 

the  bottom  of  its  burrow,  it  extends  its  slender  siphons  up 
the  tube  and  out  of  the  entrance  for  its  food  supply. 
Often  hundreds  of  individuals  enter  the  same  piece  of 
wood,  which  becomes  thoroughly  riddled  within  a  short 


FIG.  46.— The  piddock  (Zirphcea  crispata),  a  rock-boring  mollusk.     Natural  size, 

from  life. 

time,  and  though  giving  no  outward  sign  of  weakness  may 
collapse  with  its  own  weight.  Incalculable  damage  is  thus 
rendered  to  the  shipping  interests,  and  in  consequence 
much  has  been  done  to  check  their  ravages,  but  they  are 
fai  from  being  completely  overcome. 

76.  Other  stationary  species. — A  large  number  of  other 
species,  while  small  and  inconspicuous,  are  also  free  to 


MOLLUSKS 


77 


move  about,  but  as  they  become  larger  they  lose  this  ability 
either  wholly  or  periodically.  In  the  edible  mussels  (Myti- 
lus,  Fig.  47),  for  example,  which  are  associated  in  great 
numbers  on  the  rocks  along  our  coasts,  the  foot  early  be- 
comes long  and  slender  and  capable  of  reaching  out  a  con- 
siderable distance  from  the  shell  to  attach  threads  (byssus), 
which  it  spins,  to  foreign  objects.  These  are  remarkably 
strong,  and  when  several  have  been  spun  it  becomes  a  mat- 
ter of  much  difficulty  to  dislodge  them.  After  remaining 
anchored  in  one  situation  for  a  while  the  mussel  may  vol- 


Fio.  47.— The  edible  mussel  (MytUus  edulis),  showing  the  threads  by  which  it  is 
attached.    Natural  size,  from  life. 

untarily  free  itself,  and  in  a  labored  fashion  move  to  some 
other  more  favorable  spot  where  it  again  becomes  attached, 
but  there  are  numerous  species,  such  as  "  fan  shells " 
(Pinna),  scallops,  Anomia,  and  a  few  fresh-water  forms, 
where  the  union  is  permanent. 

Finally,  in  the  oysters,  some  of  the  scallops,  and  a  num- 
ber of  less  familiar  forms,  the  young  in  very  early  life  drop 
down  upon  some  foreign  object  to  which  the  shell  soon 
becomes  firmly  attached,  and  in  this  same  spot  they  pass 
the  remainder  of  their  lives.  The  oyster  usually  falls  upon 
the  left  half  of  its  shell,  which  becomes  deep  and  capacious 
enough  to  contain  the  body,  while  the  smaller  right  valve 


78  ANIMAL  FORMS. 

acts  as  a  lid.  As  locomotion  is  out  of  the  question,  the  foot 
never  develops,  and  the  shell  is  held  by  only  one  adductor 
muscle,  whose  point  of  attachment  in  the  oyster  is  indicated 
by  a  brown  scar  in  the  interior  of  the  shell. 

77.  Internal  organization. — It  is  thus  seen  that  the  ex- 
ternal features  of  the  clam  are  variously  modified,  according 
to  the  life  of  the  animal,  but  the  internal  organization  is 
much  more  uniform.  In  nearly  every  species  the  food  con- 
sists of  floating  organisms,  which  are  driven  by  the  palps 
into  the  mouth  and  on  to  the  simple  stomach,  where  it  is 
subjected  to  the  solvent  action  of  the  fluids  from  the  liver 
(Fig.  45,  B,  I)  before  entering  the  intestine.  This  latter 
structure  is  usually  of  considerable  length,  and  in  the  active 
species  extends  down  into  the  foot,  and  it  is  also  peculiar  in 
passing  through  the  ventricle  of  the  heart.  Traversing 
the  intestine  the  nutritive  portion  of  the  food  is  absorbed, 
and  is  conveyed  over  the  body  by  a  circulatory  system  more 
highly  developed  than  in  the  higher  worms.  On  the  dorsal 
side  of  the  clam,  in  a  spacious  pericardial  chamber,  the 
large  heart  is  situated  (Fig.  45,  /*),  consisting  of  a  median 
highly  muscular  ventricle  surrounding  the  intestine  and  of 
two  thin  auricles,  one  on  either  side.  From  the  former, 
two  arteries  with  their  numerous  branches  convey  the  blood 
to  all  parts  of  the  body,  where  it  accumulates,  not  in  capil- 
laries and  veins,  but  in  spaces  or  sinuses  among  the  mus- 
cles and  various  organs,  constituting  a  somewhat  indefinite 
system  of  channels  which  lead  to  the  gills  and  kidneys. 
In  these  organs  the  blood  delivers  up  the  waste  which  it 
has  accumulated  on  its  journey,  and  absorbing  a  supply  of 
oxygen,  it  flows  into  the  great  auricles,  which  in  turn  pass 
it  into  the  ventricle  to  circulate  once  more  throughout 
the  body. 

The  excretory  apparatus,  consisting  usually  of  two  kid- 
neys, of  which  one  may  degenerate  in  many  snails,  bears  a 
close  resemblance  to  that  of  the  annelids.  In  the  clam,  for 
instance,  each  consists  of  a  bent  tube  symmetrically  ar- 


MOLLUSKS  79 

ranged  on  each  side  of  the  body  (Fig.  45,  B,  &),  and  the  inner 
ends  (a),  corresponding  to  the  ciliated  funnel  of  the  anne- 
lid kidney,  open  into  the  pericardial  cavity.  The  walls 
are  continually  active  in  extracting  wastes  from  the  blood 
supplied  to  them,  and  these,  together  with  the  substances 
swept  out  from  the  pericardial  cavity,  traverse  the  tube  and 
are  carried  to  the  exterior.  In  other  mollusks  the  kidney 
may  be  more  compact,  or  greatly  elongated,  or  otherwise 
peculiar,  but  in  reality  they  bear  a  close  resemblance  to 
those  of  the  clam. 

78.  Nervous  system.— The  nervous  system,  like  the  ex- 
cretory, differs  considerably  in  different  mollusks,  yet  the 
resemblances  are  fairly  close  throughout.     In  the  clam  the 
cerebral  ganglia  corresponding  to  the  "  brain  "  in  annelids 
is  located  at  either  side,  or  above  the  mouth,  and  from  it 
several  nerves  arise,  the  larger  passing  downward  to  two 
pedal  ganglia  (p)  embedded  in  the  foot  and  to  the  visceral 
ganglia  (v)  far  back  in  the  body  (Fig.  45,  B).     These  nerve 
centers  continually  send  out  impulses  which  regulate  the 
various  activities  of  the  body  and  also  receive  impressions 
from  without.     These  come  chiefly  through  the  sense  of 
touch,  for  in  the  clams  the  other  senses  are  usually  either 
feebly  developed  or  altogether  absent. 

79.  Development. — In  the  mollusca  new  individuals  al- 
ways arise  from  eggs,  which  are  commonly  deposited  in  the 
water  and  there  undergo  development.     In  the  fresh-water 
clams  the  reproductive  organ  is  usually  situated  in  the  foot 
(Fig.  45),  while  in  the  oyster  and  similar  inactive  species  it  is 
attached  to  the  large  adductor  muscle.    In  these  latter,  and 
in  many  other  marine  forms,  the  eggs  are  shed  directly  into 
the  sea,  where  they  are  left  to  undergo  their  development 
buffeted  by  winds  and  waves  and  subject  to  the  attack  of 
numerous  enemies.     Under  such  circumstances  the  chances 
of  survival  are  slight,  and  for  this  reason  eggs  are  laid  in 
vast  numbers,  which  have  been  variously  estimated  for  the 
oyster,  for  example,  from  two  to  forty  million.    Develop- 

28 


80  ANIMAL  FORMS 

ment  proceeds  at  first  much  as  in  the  sponge,  but  soon  the 
shell,  foot,  gills,  and  various  other  molluscan  structures 
put  in  an  appearance,  and  the  few  surviving  young  which 
have  been  free-swimming  now  settle  down  in  some  favor- 
able spot,  and  attach  themselves  or  burrow  according  to 
their  habit. 

80.  Life  history  of  fresh-water  clams. — The  life  history  of 
our  common  fresh-water  clams  is  perhaps  one  of  the  most 
remarkable  known  among  mollusks.     The  parent  stores  the 
eggs,  as  soon  as  they  are  laid,  in  the  outer  gill  plate,  and 
there,  well  protected,  they  undergo  the  first  stages  of  their 
development,  which  results  in  the  formation  of  minute 
young  enclosed  in  a  bivalve  shell  beset  with  teeth.     These 
are  often  readily  obtained,  sometimes  as  they  are  escaping 
from  the  parent,  and  when  examined  under  the  microscope 
are  seen  to  rapidly  open  and  close  their  shells  in  a  snapping 
fashion  when  in  the  least  disturbed.     In  a  state  of  nature 
this  latter  movement  may  result  in  attaching  the  young  to 
the  fins  or  gills  of  some  passing  fish,  which  is  necessary  to 
its  further  development.     Within  a  short  time  it  becomes 
completely  embedded  in  the  flesh  of  its  host,  from  which, 
as  a  parasite,  it  draws  its  nourishment,  and  during  the 
next  few  weeks  undergoes  a  wonderful  series  of  transforma- 
tions resulting  in  a  small  mussel,  which  breaks  its  way 
through  the  thin  skin  of  the  fish  and  drops  to  the  bottom. 

81.  The  gasteropods. — The  gasteropods,  including  snails, 
slugs,  limpets,  and  a  host  of  related  forms,  fully  twenty 
thousand  different  species  in  all,  are  found  in  most  of  our 
fresh-water  streams  and  lakes  and  in  moist  situations  on 
land,  while  great  numbers  live  along  the  seashore  and  at 
various  depths  in  the  ocean,  even  down  as  far  as  three 
miles.     Examining  any  of  them  carefully  we  find  many  of 
the  same  organs  as  in  the  clams,  but  curiously  changed  and 
adapted  for  a  very  different  and  usually  active  life.     In  our 
common  land  snails  (Fig.  48),  which  we  may  well  examine 
before  passing  on  to  a  general  survey  of  the  group,  the  first 


MOLLUSKS 


81 


striking  peculiarity  is  in  the  univalve  shell,  with  numerous 
whorls,  into  which  the  animal  may  at  any  time  withdraw 
completely.  Ordinarily  this  is  carried  on  the  back  of  the 
spindle-shaped  body,  which  is  fashioned  beneath  into  a  great 


FIG.  48.— The  slug  (Ariolimax)  and  common  snail  (Helix).    From  life. 

flat  sole  or  creeping  surface  that  bears  on  its  forward  bor- 
der a  wide  opening  through  which  mucus  is  continually 
issuing  to  enable  the  snail  to  slip  along  more  readily.  Slime 
also  exudes  on  other  points  on  the  surface  of  the  body  and 
affords  a  valuable  protection  against  excessive  heat  and 
drought. 

Unlike  the  clams,  the  forward  end  of  the  body  is  devel- 
oped into  a  well-marked  head  bearing  the  mouth  and  a 
complicated  mechanism  for  gathering  and  masticating  food, 
together  with  two  pairs  of  tentacles,  one  of  which  carries  the 
eyes.  On  the  right  side  of  the  animal,  some  distance  behind 
the  head,  is  the  opening  of  the  little  sac-like  mantle  cavity 
(Fig.  48)  which  contains  the  respiratory  organs,  and  into 
which  the  alimentary  canal  and  the  kidneys  pour  their 
wastes.  The  relation  of  these  organs  to  the  mantle  cavity 
is  the  same  as  in  the  clams,  though  the  cavities  differ  much 
in  size  and  position. 

82.  Other  snails.  The  shell. — Extending  our  acquaint- 
ance to  other  species  of  snails,  we  find  the  same  general 
plan  of  body,  although  somewhat  obscured  at  times  by 


ANIMAL  FORMS 


FIG.  49.— The  chiton,  armadillo-snail  or  sea-era  - 
dle.     The  left-hand  figure  shows  mouth  in 


many  modifications.  A  foot  is  generally  present,  also  a 
more  or  less  well-developed  head,  and  the  body  is  usually 
surrounded  by  a  shell  which  varies  widely  in  shape  and 
size  in  different  species.  In  the  common  limpets  the  early 
coiled  shell  is  transformed  into  an  uncoiled  cap-like  one, 
and  in  the  keyhole  limpets  is  perforated  at  its  summit.  The 

chitons  or  armadillo- 
snails  (Fig.  49),  often 
found  associated  with 
the  limpets,  carry  a 
most  peculiar  shell  con- 
sisting of  eight  plates, 
which  enables  the  ani- 
mal to  roll  up  like  an 
armadillo  when  dis- 
turbed. A  shell  is  by 

center  of  proboscis,  the  broad  foot  on  each  n°     means    a    necessity, 

side  of  which  are  numerous  small  gills.  The  however,    f Or     in     many 
right-hand  figure  shows  the  mantle  and  shell,  •  h  fh 

composed  of  eight  plates.     From  life,  one-  bPe(    es> 

half  natural  size.  beautiful  naked  snails 

or  Nudibranchs  (Fig. 

50)  common  along  our  coasts,  it  may  be  entirely  absent, 
or,  as  in  the  ordinary  slugs,  reduced  to  a  small  scale  em- 
bedded in  the  skin. 

83.  Respiration, — A  considerable  quantity  of  oxygen  is 
absorbed  through  the  skin,  as  in  all  mollusks,  but  the  chief 
part  of  the  process  is  usually  taken  by  the  plume-like  gills, 
one  or  two  in  number,  which  are  located  in  the  mantle 
cavity.  In  the  chitons  (Fig.  49)  the  number  of  gills  is 
greater,  amounting  in  some  species  to  over  a  hundred, 
while  in  the  Nudibranchs  (Fig.  50)  gills  are  absent,  their 
places  being  taken  by  more  or  less  feathery  expansions  of 
the  skin  on  the  dorsal  surface. 

Many  of  the  gasteropods  left  exposed  on  the  rocks  by  a 
retreating  tide  retain  water  in  the  mantle  cavity,  from 
which  they  extract  the  oxygen  until  submerged  again. 


MOLLUSKS  83 


Others  breathe  by  means  of  gills  while  under  water,  and  by 
the  surface  of  the' body  and  the  moist  walls  of  the  mantle 


FIG.  50.— Three  different  species  of  naked  marine  snails  or  Nudibranchs.    Natural 
size,  from  life. 

cavity  when  exposed.  In  some  of  the  small  Littorinas 
attached  so  far  from  the  sea  as  to  be  only  occasionally 
washed  by  the  surf  this  latter  method  may  prevail  for  days 
together — in  fact  they  live  better  out  of  water  than  in  it. 
It  is  not  difficult  to  imagine  that  such  forms,  keeping  in 
moist  places,  might  wander  far  from  the  sea,  and,  losing 
their  gills,  become  adapted  to  a  terrestrial  life.  It  is 
believed  that  in  past  times  this  has  actually  occurred,  and 
that  our  land  forms  trace  their  descent  from  aquatic  ances- 
tors. To-day  they  breathe  by  a  lung — that  is,  they  take 
oxygen  through  the  walls  of  the  mantle  cavity,  as  the  slug 
may  be  seen  to  do,  though  in  some  species  traces  of  the  old 
gill  yet  remain. 

84.  Food  and  digestive  system. — Many  mollusks  live  upon 
seaweeds,  and  the  greater  number  of  terrestrial  forms  are 
fond  of  garden  vegetables  or  certain  kinds  of  lichens,  but, 
on  the  other  hand,  the  latter,  together  with  a  large  number 
of  marine  snails,  are  carnivorous.  In  all  cases  the  food 
requires  to  be  masticated,  and,  unlike  the  clams,  the  mouth 
is  usually  provided  with  horny  jaws,  and  an  additional 


ANIMAL  FORMS 


FIG.  51.— A  small  portion  of  the  radula  or 
tongue-rasp  of  a  snail  (Sycotypus). 


masticatory  apparatus  which  consists  of  a  kind  of  tongue 
with  eight  to  forty  thousand  minute  teeth  in  our  land 
forms  (Fig.  51),  while  in  certain  marine  snails  they  are 
beyond  computation.  With  the  licking  motion  of  the 
tongue  this  rasp  tears  the  food  into  shreds  before  it  is 
swallowed,  and  in  the  whelks  or  borers  it  serves  to  wear  a 
circular  hole  through  the  shells  of  other  mollusks,  which 

are  thus  killed  and  devoured. 
This  latter  process  is  facili- 
tated by  the  secretion  of  the 
salivary  glands,  which  has  a 
softening  effect  upon  the 
shell.  Ordinarily  the  saliva 


of  snails  exercises  some  di- 
gestive action. 

In  the  stomach  of  some 
snails  are  teeth  or  horny 
ridges  which  also  are  instrumental  in  crushing  the  food, 
and  in  numerous  minor  respects  peculiarities  exist  in  differ- 
ent species  according  to  the  nature  of  the  food ;  but  in  its 
general  features  the  digestive  tract  is  similar  to  that  of 
the  clams. 

The  processes  of  circulation  and  excretion  are  also  car- 
ried on  by  means  of  systems  which  show  a  certain  resem- 
blance to  those  of  the  clams.  As  might  be  expected,  certain 
differences  exist,  sometimes  very  great,  but  they  are  of  too 
technical  a  nature  to  concern  us  further. 

85.  Sense-organs  of  lamellibranchs  and  gasteropoda.— 
The  eyes  of  mollusks  differ  widely  in  their  structure  and 
the  position  they  occupy  in  the  body.  In  our  common 
land  snails  two  pairs  of  tentacles  are  borne  on  the  head, 
the  lower  acting  as  feelers,  while  each  of  the  upper  ones 
bears  on  its  extremity  the  eye,  appearing  as  a  minute  black 
dot  (Fig.  48).  In  this  same  position  the  eyes  of  many 
marine  snails  occur,  but  there  are  numerous  species  in 
which  there  are  other  accessory  eyes.  In  many  of  the 


MOLLUSKS  85 

limpets,  for  instance,  there  are  numbers  of  additional  eyes 
carried  on  the  mantle  edge  just  under  the  eaves  of  the 
shell,  and  forming  a  row  completely  encircling  the  body. 
(In  the  scallops  there  are  two  rows  of  brilliantly  colored 
eyes,  set  like  jewels  on  the  edges  of  the  mantle  just  within 
the  halves  of  the  shell.)  In  the  chitons  the  eyes  of  the 
head  disappear  by  the  time  the  animal  attains  maturity, 
and  in  some  species  at  least  their  place  appears  to  be  taken 
by  great  numbers  of  eyes,  sometimes  thousands,  which  are 
embedded  in  the  shells.  On  the  other  hand,  eyes  are  com- 
pletely absent  in  certain  species  of  burrowing  snails  and  in 
several  living  in  the  gloomy  depths  of  the  sea  far  from  the 
surface ;  they  appear  to  be  absent  also  from  fresh-water 
clams ;  but  the  fact  that  certain  species  close  their  shell 
when  a  shadow  falls  upon  them,  leads  to  the  belief  that 
while  actual  eyes  are  not  present  the  skin  is  extremely 
sensitive  to  light.  This  is  also  the  case  with  many  snails. 

86.  Smell. — Since  the  sense  of  sight  is  generally  unde- 
veloped in  the  mollusks,  they  rely  chiefly  upon  touch  and 
smell  for  recognizing  the  presence  of  enemies  and  food. 
Tentacles  upon  the  head  and  other  parts  of  the  body,  and 
a  skin  abundantly  supplied  with  nerves,  show  them  to  pos- 
sess a  high  degree  of  sensibility ;  but  in  the  greater  num- 
ber of  species  the  sense  of  smell  is  of  chief  importance. 
Many  experiments  show  that  tainted  meat  and  strongly 
scented  vegetables  concealed  from  sight  and  several  feet 
distant  from  many  of  our  land  and  sea  mollusks  will  attract 
them  at  once.    In  these  forms  the  sense  of  smell  appears  to 
be  located  on  the  tentacles,  but  additional  organs,  possibly 
of  smell,  are  located  on  various  portions  of  the  body,  usu- 
ally in  the  neighborhood  of  the  gills. 

87.  Taste  and  hearing. — Several  mollusks  appear  to  be 
almost  omnivorous,  but  others  are  decidedly  particular  in 
their  choice  of  food,  which  leads  us  to  suspect  that  they 
possess  to  some  extent  the  sense  of  taste.     Nerves  supply- 
ing the  base  of  the  mouth  have  also  been  detected,  which 


86  ANIMAL  FORMS 

may  be  those  of  taste ;  but  experiments  along  the  line  are 
difficult  to  perform,  and  our  knowledge  of  this  subject  is 
far  from  complete.  The  same  is  true  of  hearing.  Certain 
organs,  interpreted  as  ears  and  located  in  the  foot,  have 
the  form  of  two  hollow  sacs,  containing  one  or  more  solid 
particles  of  sand  or  lime,  whose  jarrings,  when  effected  by 
sonorous  bodies,  may  result  in  hearing.  On  the  other  hand, 
it  is  held  by  some  that  they,  like  the  semicircular  canals  of 
higher  animals,  may  regulate  the  muscular  movements 
which  enable  the  animal  to  keep  its  balance. 

88.  Egg-laying  habits  and  development.— The  egg-laying 
habits  of  the  gasteropods  differ  almost  as  widely  as  their 
haunts.     The  terrestrial  forms  lay  comparatively  few  eggs, 
ranging  in  size  from  small  shot  to  a  pigeon's  egg  in  some 
of  the  tropical  species.     These  are  buried  in  hollows  in  the 
ground  or  under  sticks  and  stones,  and  after  a  few  weeks 
hatch  out  young  snails  having  the  form  of  the  adult.    The 
same  is  also  true  of  most  of  the  fresh-water  snails,  which 
lay  relatively  smaller  eggs  embedded  in  a  gelatinous  mass 
frequently  found  attached  to  sticks  and  leaves,  or  on  the 
walls  of  aquaria  in  which  they  are  confined.     Many  marine 
species   construct   capsules   of  the   most   varied   patterns 
which  they  attach  to  different  objects,  and  in  these  the 
young  are  protected  until  they  hatch.     In  the  limpets  and 
many  of  the  chitons  the  eggs  are  laid  by  thousands  directly 
in  the  water,  and  after  a  short  time  develop  into  free-swim- 
ming young,  differing  considerably  from  the  parent  in  ap- 
pearance.   Those  escaping  the  ravages  of  numerous  enemies 
finally  settle  down  in  a  favorable  situation  and  gradually 
assume  the  form  of  the  adult. 

89.  Age,  enemies,  and  means  of  defense  of  lamellibranchs 
and  gasteropods. — How  much  time  is  consumed  by  the  young 
in  growing  up,  and  the  length  of  time  they  live,  are  ques- 
tions generally  unsettled.    It  is  said  that  the  oyster  requires 
five  years  to  attain  maturity,  and  lives  ten  years  ;  the  fresh- 
water clam  develops  in  five  years,  and  some  species  live  from 


MOLLUSKS  87 

twelve  to  thirty  years ;  and  the  average  length  of  life  of 
the  snail  appears  to  be  from  two  to  five  years.  Certain  it 
is  that  mollusks  have  numerous  enemies  besides  man  which 
prevent  multitudes  from  living  lives  of  normal  length. 
Birds,  fishes,  frogs,  .and  starfishes  beset  them  continually, 
and  many  fall  a  prey  to  the  ravages  of  internal  parasites  or 
to  other  mollusks.  Under  ordinary  circumstances  the  shell 
is  sufficient  protection,  and  the  spines  disposed  on  the  sur- 
face in  many  species  render  the  occupant  still  less  liable  to 
attack.  Many  snails  carry  on  the  foot  a  horny  or  calcare- 
ous plate  known  as  the  operculum,  which  closes  the  en- 
trance of  the  shell  like  a  door  against  intruders.  Certain 
noxious  secretions  poured  out  from  the  skin  also  serve  as  a 
means  of  defense,  and  many  Nudibranchs  (Fig.  50)  bear 
nettle- cells  on  the  processes  of  the  body,  which  probably 
render  them  distasteful  to  many  animals.  Finally,  there 
are  numerous  clams,  mussels,  snails,  and  slugs  whose  colors 
harmonize  so  closely  with  their  surroundings  that  they  al- 
most completely  baffle  detection,  and  enable  them  to  lead 
as  successful  a  life  as  those  provided  with  special  organs  of 
defense. 

90.  Cephalopods. — The  animals  belonging  to  this  class, 
such  as  the  squids  and  cuttlefishes  (Fig.  52),  are  by  far 
the  most  highly  developed  mollusks.  They  are  of  great 
strength,  capable  of  very  rapid  movements,  and  several  spe- 
cies are  many  times  the  largest  invertebrates.  In  almost 
every  case  there  is  a  well-defined  head  bearing  remarkably 
perfect  eyes,  and  also  a  circle  of  powerful  arms  provided 
with  numerous  suckers  which  aid  in  the  capture  of  food 
(Fig.  52).  Posteriorly  the  body  is  developed  into  a  pointed 
or  rounded  visceral  mass  which  to  a  certain  extent  is  free 
from  the  head,  giving  rise  to  a  well-marked  neck.  Some 
forms,  such  as  the  squids  (Fig.  52,  upper  figure),  are  pro- 
vided with  fins  which  drive  the  animal  forward,  but  in  com- 
mon with  other  cephalopods  they  are  capable  of  a  very  rapid 
backward  motion.  By  muscular  movements  water  is  taken 


88 


ANIMAL  FORMS 


into  the  large  mantle  cavity  within  the  body,  a  set  of  valves 
prevents  its  exit  through  the  same  channels,  and  upon  a 
Vigorous  contraction  of  the  body  walls  the  water  is  forced 
Out  rapidly  through  the  small  opening  of  the  funnel,  which 


FIG.  52.— Cephalopoda.  Lower  figure,  the  devil-fish  or  octopus  ( Octopus  punctatus). 
The  upper  figure  represents  the  squid  (Loligo  pealii)  swimming  backward  by 
driving  a  stream  of  water  through  the  small  tube  slightly  beneath  the  eyes.  Prom 
life,  one-third  natural  size. 

drives  the  animal  backward  after  the  fashion  of  an  explod- 
ing sky-rocket.  In  this  way  they  usually  escape  the  fishes 
and  whales  that  prey  upon  them,  but  an  additional  device 
has  been  provided  in  the  form  of  a  sac  within  the  body, 
whose  inky  contents  may  be  liberated  in  such  quantity  as 
to  cloud  the  water  for  a  considerable  distance,  and  thus 
enable  them  to  slip  away  unseen  into  some  place  of  safety. 
Most  of  the  cephalopods  are  further  protected  by  their 
ability  to  assume,  like  the  chameleon,  the  color  of  the  object 


MOLLUSKS  89 

upon  which  they  rest.  In  the  skin  are  embedded  multi- 
tudes of  small  spherical  sacs  filled  with  pigments  of  various 
colors,  chiefly  shades  of  red,  brown,  and  blue,  each  sac  be- 
ing connected  with  a  nerve  and  a  series  of  delicate  muscles. 
If  the  animal  settles  upon  a  red  surface,  for  example,  a 
nerve  impulse  is  sent  to  each  of  the  hundreds  of  color  sacs 
of  corresponding  shade,  causing  the  muscles  to  contract 
and  flatten  the  bag  like  a  coin,  and  thus  exposing  a  far 
greater  surface  than  before,  they  give  the  animal  a  reddish 
hue.  In  the  twinkling  of  an  eye  they  may  completely 
change  to  another  tint,  or  present  a  mottled  look,  and  some 
may  even  throw  the  surface  of  the  skin  into  numerous 
small  projections  that  make  the  animal  appear  part  of  the 
rock  upon  which  it  rests.  These  devices  not  only  serve  for 
protection,  but  they  also  aid  in  enabling  these  mollusks  to 
steal  upon  their  prey,  chiefly  fishes,  which  they  destroy  in 
great  numbers  with  lionlike  ferocity. 

The  devil-fishes  and  a  number  of  other  species  are  usu- 
ally found  creeping  along  the  sea  bottom,  generally  near 
shore,  and  are  solitary  in  their  habits,  while  the  squids  re- 
main near  the  surface  and  frequently  travel  in  great  com- 
panies, sometimes  numbering  hundreds  of  thousands.  In 
size  they  usually  range  from  a  few  inches  to  a  foot  or  two 
in  length,  but  a  few  devil-fishes  and  squids  attain  a  greater 
size,  some  of  the  latter  reaching  the  enormous  length  of 
from  forty  to  sixty  feet.  There  are  many  stories  of  their 
great  strength  and  of  their  voluntarily  attacking  people 
and  even  overturning  boats,  but  the  latter  are  in  almost 
every  case  sailors'  yarns. 

In  their  external  organization  the  cephalopods  have 
little  to  remind  one  of  any  of  the  preceding  mollusks,  and 
their  internal  structure  shows  only  a  distant  resemblance. 
In  the  Octopi  (Fig.  52)  the  shell  is  lacking  ;  in  the  squid  it 
is  called  the  pen,  and  consists  of  a  horn-like  substance  with- 
out any  lime  deposit ;  in  the  cuttlefishes  it  is  spongy  and 
plate-like,  and  is  a  familiar  object  in  the  shops  ;  and,  finally, 


90  ANIMAL  FORMS 

in  the  nautilus  it  is  coiled  and  of  considerable  size,  and,  un- 
like that  of  any  other  cephalopod,  it  is  carried  on  the  out- 
side of  the  animal.  Interiorly  it  is  divided  by  a  number  of 
partitions  into  chambers,  the  last  one  of  which  is  occupied 
by  the  animal. 

The  alimentary  canal  shows  some  resemblance  to  that 
of  other  mollusks,  but,  as  in  the  case  of  the  other  systems 
of  the  body,  it  possesses  a  far  higher  state  of  development. 
The  mouth  is  situated  in  the  center  of  a  circle  of  arms, 
which  in  reality  are  modified  portions  of  the  foot,  and  is 
furnished  with  two  parrot-like  jaws.  From  this  point  the 
esophagus  leads  back  into  the  body  mass  to  the  stomach, 
which  with  the  liver  and  intestine  are  sufficiently  like 
those  of  the  clam  and  snail  to  require  no  further  comment. 

Eespiration  is  effected  by  the  skin  to  a  certain  extent, 
but  chiefly  by  two  gills  (four  in  the  nautilus),  and  the  cir- 
culatory system,  which  conveys  the  blood  to  and  from  these 
organs  and  over  the  body  with  its  complex  heart,  arteries, 
capillaries,  and  veins,  is  more  highly  developed  than  in 
any  other  invertebrate. 

As  might  be  expected  in  animals  with  so  great  sagacity 
and  cunning,  the  nervous  system  and  the  sense-organs  reach 
a  degree  of  development  but  little  short  of  what  we  find  in 
some  of  the  vertebrates.  The  chief  part  of  the  nervous 
system  is  located  in  the  head,  protected  by  a  cartilaginous 
skull,  a  very  rare  structure  among  invertebrates ;  and  while 
the  different  ganglia  may  be  recognized  in  a  general  way 
and  be  found  to  correspond  to  a  certain  extent  to  those 
of  foregoing  mollusks,  they  are  so  largely  developed  and 
massed  together  that  it  is  impossible  at  present  to  under- 
stand them  fully.  From  this  point  nerves  pass  to  all 
regions  of  the  body,  to  the  powerful  muscles,  the  viscera, 
and  the  organs  of  special  sense,  controlling  the  complex 
mechanism  in  all  its  workings. 

There  is  no  doubt  that  the  cephalopods  see  distinctly 
for  considerable  distances,  and  a  careful  examination  of 


MOLLUSKS  91 

the  eye  of  the  squids  and  cuttlefishes  has  shown  them 
to  be  remarkably  complex  and  in  many  respects  to  be 
constructed  upon  much  the  same  plan  as  those  of  the 
vertebrates.  As  to  the  other  senses  not  so  much  is  known, 
but  undoubtedly  many  species  of  cephalopods  are  possessed 
of  a  shrewdness  and  cunning  not  shared  by  any  other 
invertebrates,  save  some  of  the  insects  and  spiders,  and  are 
vastly  more  highly  organized  than  their  molluscan  rela- 
tives. 

91.  How  species  originate.— We  have  now  examined  a 
considerable  portion  of  the  animal  kingdom,  tracing  its 
members  from  their  simplest  beginnings  as  single  cells, 
through  the  formation  of  colonial  types,  and  up  through 
the  sponges,  coelenterates,  worms,  and  mollusks.  It  is  im- 
portant once  more  to  note  that  they  all  perform  the  func- 
tions concerned  in  nutrition  and  reproduction,  and  only 
these.  The  differences  which  exist  are  those  of  structure. 
The  Hydra  and  the  clam,  for  example,  perform  the  same 
duties,  but  their  bodily  apparatus  differs  widely,  and  the 
completeness  and  perfection  of  the  work  varies  accord- 
ingly. The  more  the  work  to  be  performed  by  an  organ- 
ism is  divided  up  among  especially  adapted  organs,  so  that 
each  of  the  latter  has,  as  far  as  possible,  only  one  thing  to 
do,  the  higher  is  the  organism. 

As  stated  earlier  in  the  account,  it  is  believed  that  ihe 
more  complex  animals  arose  from  the  simpler ;  that  if  we 
could  trace  the  history  of  any  of  the  great  groups  back 
toward  their  first  beginnings,  we  would  find  them  all  to 
have  originated  from  one  ancestral  form,  that  in  turn  owes 
its  descent  from  yet  simpler  forms. 

Let  us  see  something  of  how  this  has  come  about.  We 
all  know  that  vast  numbers  of  young  are  born  into  this 
world  which  never  come  to  maturity.  It  is  said  that  if  all 
the  young  of  the  codfish  were  to  live  their  allotted  time, 
it  would  be  less  than  fifteen  years  before  the  sea  would 
become  literally  packed  with  them.  Numerous  enemies, 


92  ANIMAL  FORMS 

the  lack  of  food,  and  other  agencies  annihilate  the  larger 
part.  We  also  know  that  no  two  offspring  are  exactly 
alike.  They  exhibit  individual  differences.  One  bird  may 
have  a  larger  bill  than  another  of  the  same  brood  which 
excels  in  length  of  wing.  As  noted  above,  all  the  offspring 
will  not  attain  maturity.  Those  best  adapted  to  their  sur- 
roundings will  have  the  best  chances  of  survival.  The 
increased  length  of  bill  or  wing  may  be  slight,  but  it  may 
be  just  this  amount  which  enables  the  bird  to  probe  deeper 
or  fly  farther  and  thus  secure  the  requisite  amount  of  food. 
A  premium  is  placed  on  length  of  wing  or  bill  generation 
after  generation,  with  the  result  that  a  long-billed  species 
arises  distinct  from  the  long-winged  which  trace  their 
ancestry  back  to  the  same  parents.  It  is  the  same  prin- 
ciple which  enables  the  breeder  to  increase  the  swiftness 
of  the  race-horse  and  the  strength  of  the  draft-horse,  or 
the  gardener  to  develop  from  the  wild  rose  the  great  num- 
ber of  widely  different  varieties.  In  the  same  way  other 
slight  peculiarities  over  very  many  generations  may  en- 
able other  forms  to  gradually  adapt  themselves  to  still  dif- 
ferent modes  of  life.  Thus  vast  numbers  of  organisms 
gradually  become  modified  in  form  and  complexity,  and 
are  adapted  to  lives  which  insure  them  a  comparative 
degree  of  safety  and  less  competition  with  other  species. 

The  above  account  serves  solely  for  purposes  of  illustra- 
tion. No  kind  of  bird  has  originated  in  just  that  way,  but 
as  the  essential  force  in  all  change  of  form  we  have  the 
necessity  of  adaptation  of  the  individual  to  its  surround- 
ings, the  death  of  those  who  can  not  be  adapted,  and  the 
inheritance  of  the  advantage  of  the  parent  by  its  progeny, 
enabling  these  in  turn  to  survive  and  to  multiply  their 
own  kind. 


CHAPTER  IX 

ARTHROPODS.   CLASS  CRUSTACEA 

92.  General  characters. — In  the  Arthropods,  that  is,  the 
crabs,  lobsters,  shrimps,  insects,  spiders,  and  a  vast  host 
of  related  forms,  the  body  is  bilaterally  symmetrical,  and 
is  composed  of  a  number  of  segments  arranged  in  a  series, 
as  in  the  earthworm  and  other  annelids.     A  hornlike  cu- 
ticle, sometimes  called  the  shell,  bounds  the  external  sur- 
face— in  early  life  thin  and  delicate,  but  later  relatively 
thick,  and  often  further  strengthened  by  lime  salts.   Along 
the  line  between  the  segments  this  coat  of  mail  remains 
thin  and  forms  a  flexible  joint.    Appendages  also  are  borne 
on  each  segment,  not  comparatively  short  and  fleshy  out- 
growths like  the  lateral  appendages  of  many  of  the  worms, 
but  usually  long  and  jointed  (hence  the  name  Arthropod, 
meaning  jointed  foot),  and  variously  modified  for  many 
different  uses. 

93.  Classification. — The  species  belonging  to  this  group 
outnumber  the  remainder  of  the  animal  kingdom.     Their 
haunts  also  are  most  diverse.     Some  are  adapted  for  lives 
in  the  sea  and  fresh  water,  others  for  widely  different  sit- 
uations on  land,  and  a  great  number  are  constructed  for  a 
life  on  the  wing.    A  certain  resemblance  exists  among  them 
all,  but  the  modifications  which  fit  them  for  their  different 
habitats  are  also  profound,  and  have  resulted  in  the  division 
of  the  Arthropods  into  five  classes.     The  first  class  ( Crus- 
tacea) contains  the  crayfish,  crabs,  etc. ;  the  second  ( Ony- 
chophora)  includes  the  curious  worm-like  peripatus  (Fig. 


I 


94  ANIMAL  FORMS 

66) ;  the  third  (Myriapoda,  meaning  myriad-footed)  em- 
braces the  centipeds  and  "  thousand-legs  "  ;  the  fourth  (In- 
secta)  contains  the  insects ;  and  the  fifth  (Arachnida)  in- 
cludes the"  scorpions,  spiders,  and  mites. 

94.  The  Crustacea. — The   number  of  species  of  crusta- 
ceans is  estimated  to  be  about  ten  thousand,  and  while  the 
greater  number  of  these  are  marine,  many  are  found  in 
fresh  water  and  a  few  occur  on  land.     In  size  they  range 
from  almost   microscopic   forms  to  the   giant  crabs   and 
lobsters.     They  differ  also  in  shape  to  a  remarkable  degree, 
but  at  the  same  time  there  is  a  decided  resemblance  through- 
out the  group,  except  in  those  species  which  have  become 
modified  by  a  parasitic  habit.     The  characteristic  external 
skeleton  is  invariably  present,  and  gives  evidence  of  the 
deep  internal  segmentation   of  the  body.     In  the  simple 
Crustacea  this  is  very  apparent,  but  in  the  higher  forms  it 
is  usually  more  or  less  obscured,  owing  to  the  fusion  of  some 
of  the  different  segments,  especially  those  of  the  head,  as  in 
the  crayfish  (Fig.  59). 

The  class  of  the  Crustacea  is  subdivided  into  two  sub- 
classes (Entomostraca  and  Malacostraca),  the  first  containing 
the  fairy-shrimps  (Brancliipus,  Fig.  53)  and  their  allies,  the 
copepods  (such  as  Fig.  54),  the  barnacles  (Fig.  55),  and  a 
number  of  other  species.  In  their  organization  all  are  com- 
paratively simple,  usually  small,  and  the  appendages  show 
relatively  little  specialization.  The  other  subclass  contains 
the  more  highly  developed  and  usually  large-sized  Crustacea, 
among  which  are  the  shrimps,  crayfishes,  lobsters,  crabs, 
and  a  number  of  other  forms. 

95.  Some  simple  Crustacea. — While  the  members  of  the 
first  subclass  are  minute  and  inconspicuous,  several  species 
are  often  remarkably  abundant  in  our  small  fresh-water 
pools.    Among  these  is  the  beautifully  colored  fairy-shrimp 
(BrancMpus,  Fig.  53),  with  greatly  elongated  body  and 
leaf-like  appendages,  whose  relatively  simple  character  leads 
the  zoologist  to  think  that  they  are  among  the  simplest 


ARTHROPODS.      CLASS  CRUSTACEA  95 

Crustacea,  and  in  several  points  resemble  the  ancestral  form 
from  which  all  the  modern  species  have  descended.  Some 
nearly  related  forms  are  provided  with  a  great  fold  of  the 
body-wall,  which  may  almost  completely  conceal  the  animal 
from  above,  or  it  may  be  formed  like  a  bivalve  clam-shell, 
within  which  the  entire  body  may  be  withdrawn.  This 

ft 


FIG.  53.— Fairy-shrimp  (Branchipus).  b,  brood-pouch ;  e,  e*, 
compound  and  simple  eyes  ;  f,  paddle-shaped  feet ;  h,  tu- 
bular heart ;  i,  intestine. 


latter  character  is  also  found  in  the  water-fleas  (Daplinia), 
very  much  smaller  forms,  and  sometimes  occurring  in  mil- 
lions on  the  bottoms  of  our  ponds  and  marshes.  They  are 
readily  distinguished  from  the  fairy-shrimp  by  the  short- 
ness of  the  body,  the  small  number  of  appendages,  and  by 
their  habit  of  using  their  antennae  as  swimming  organs, 
which  gives  to  their  locomotion  a  jerky,  awkward  character. 
96.  Cyclops  and  relatives. — Cyclops  (Fig.  54),  the  repre- 
sentative of  a  number  of  lowly  forms  belonging  to  the  order 
of  Copepods,  is  one  of  the  commonest  fresh-water  Crustacea. 
The  forward  segments  of  the  spindle-shaped  body  are  cov- 
ered by  a  large  shield  or  carapace,  the  feet  are  few  in  num- 
ber, and,  like  its  fabled  namesake,  it  bears  an  eye  in  the 
center  of  the  forehead.  Nearly  related  species  are  also  re- 
markably abundant  at  the  surface  of  the  sea,  at  times  occur- 
29 


96 


ANIMAL  FORMS 


ring  in  such  vast  numbers  that  they  impart  a  reddish  tinge 
to  the  water  over  wide  areas,  and  at  night  are  largely  re- 
sponsible for  its  phos- 
phorescence. Many  oth- 
ers are  parasitic  in  their 
habits,  and  scarcely  a 
salt-water  fish  exists  but 
that  at  one  time  or  an- 
other suffers  from  their 
attacks.  On  the  other 
hand,  many  fresh-  and 
salt-water  fishes  depend 
upon  the  free-swimming 
forms  for  food,  and 
hence,  from  an  economic 
point  of  view,  they  are 
highly  important  organ- 
isms. 

97.  Barnacles.  —  The 
parasitic  habit  and  the 
lack  of  locomotion  has 
also  produced  marvelous 
changes  among  the  bar- 
nacles, so  great  that 
originally  they  were 
placed  among  the  mol- 
lusks ;  and  as  with  the  parasitic  copepods,  their  true  posi- 
tion was  only  known  after  their  life-history  had  been  de- 
termined. In  the  goose-barnacles  *  the  body,  attached  by 
a  fleshy  stalk  to  foreign  objects,  is  enclosed  by  a  tough 
membrane,  corresponding  to  the  carapace  of  other  Crus- 
tacea, in  which  are  embedded  five  calcareous  plates.  This 


FIG.  54.— Cyclops,    e.  s.,  eggs ;  i,  intestine 
reproductive  organ. 


ov, 


*  So  called  because  of  the  belief,  which  existed  for  three  hundred 
years  prior  to  the  present  century,  that  when  mature  these  animals 
give  birth  to  geese. 


ARTHROPODS.     CLASS  CRUSTACEA 


97 


is  open  along  one  side,  and  allows  the  feather-like  feet  to 
project  and  produce  currents  in  the  surrounding  water 
which  brings  food  within  reach.  In  the  acorn-barnacles 
(Fig.  55)  the  stalk  is  absent,  and  the  body,  though  possess- 


FIG.  55.— Barnacles.    Acorn-barnacles  chiefly  in  lower  part  of  figure  ;  goose-barnacles 
above.    Natural  size. 

ing  the  same  general  character  as  the  goose-barnacles,  is 
shorter,  and  enclosed  in  a  strong  palisade  consisting  of  six 
calcareous  plates. 

The  larger  number  of  barnacles  attach  themselves  to 
the  supports  of  wharves,  the  hulls  of  ships,  floating  tim- 
bers, the  rocks  from  the  shore-line  down  to  considerable 
depth,  and  a  few  species  occur  on  the  skin  of  sharks  and 
whales.  On  the  other  hand,  there  are  several  species  which 
are  parasitic,  and  in  accordance  with  this  mode  of  life  ex- 
hibit various  degrees  of  degeneration.  In  the  most  extreme 


98  ANIMAL  FORMS 

cases  (Sacculind)  the  sac-like  body,  attached  to  the  abdo- 
men of  crabs,  is  entirely  devoid  of  appendages  and  any 
signs  of  segmentation.  A  root-like  system  of  delicate  fila- 
ments extends  from  the  exposed  part  of  the  animal  into 
the  host  and  absorbs  the  necessary  nutriment.  The  mouth 
and  alimentary  canal  are  accordingly  absent — in  fact,  the 
body  contains  little  but  the  reproductive  organs  and  a  very 
simple  nervous  system. 

98.  Structure. — In  the   internal    organization  of  these 
smaller  crustaceans  many  differences  may  be  noted,  though 
they  are  usually  less  profound  than  the  external.     Ordi- 
narily the   alimentary   canal    is   a   straight   tube   passing 
through    the    body,   and    is   provided   with    a    pouch-like 
stomach,  and   a   more  or  less   clearly  defined   liver.     In 
all,  except  the  parasitic  species,  the  external  mouth-ap- 
pendages masticate  the  food,  and  in  a  very  few  of  the 
above-described  groups  it  may  be  further  ground  between 
the  horny  ridges  on  the   stomach-walls.     After  this  pre- 
liminary treatment  it  is  subjected  to  the  action  of  the 
digestive  juices,  and  when  liquefied  is  absorbed  into  the 
body.     Here  it  is  circulated  by  a  blood-system  of  widely 
different  character.     In  many  cases  definite  arteries  and 
veins  are  absent.     The  blood  courses  through  the  body  in 
the  spaces  between  the  different  organs  propelled  by  the 
beating  of  the  heart,  which  it  is  made  to  traverse.     In 
Cyclops  (Fig.  54)  even  the  heart  is  absent,  and  the  blood 
is  made  to  circulate  by  contractions  of  the  intestine.     In 
most  of  these  smaller  Crustacea  considerable  oxygen  is  ab- 
sorbed through  the  body-wall ;  but  in  several  species,  for 
example,  the  fairy-shrimp  (Fig.  53),  special  gills  are  devel- 
oped on  the  appendages  of  the  body. 

99.  Multiplication. — Among  the  Crustacea  thus  far  con- 
sidered the  males  are  usually  readily  recognized  owing  to 
their  small  size.     The  females  also  are  usually  provided 
with  brood-pouches  in  which  the  developing  eggs  are  pro- 
tected.    In  almost  every  case  the  young  are  born  in  the 


ARTHROPODS.     CLASS  CRUSTACEA 


99 


form  of  minute  larvae,  provided  with  three  pairs  of  append- 
ages, a  median  eye  (Fig.  56),  and  a  firm  external  skeleton 
or  cuticle.  This  latter  prevents  the  continuous  growth  of 
the  larvae  or  nauplius,  and  every  few  days  it  is  thrown  off, 
and  while  the  new  one  is  forming  the  body  enlarges.  Dur- 
ing this  time  new  appendages  are  developed,  so  that  after 
each  moult  the  young  crusta- 
cean emerges  less  like  its 
former  self  and  more  and  more 
like  its  parents.  In  the  bar- 
nacles, after  several  moults 
have  taken  place,  the  young 
become  permanently  attached 
by  means  of  their  first  anten- 
nae, their  thoracic  feet  change 
into  feathery  appendages,  and 
several  other  changes  occur. 
In  some  of  the  parasitic  bar- 
nacles (Sacculina)  the  larva 
attaches  itself  to  a  crab,  throws 
off  its  various  appendages,  and, 
after  other  great  degenerative 
changes,  enters  its  host.  For 
a  time,  therefore,  their  development  is  toward  greater  com- 
plexity, but  the  later  stages  constitute  a  retrograde  meta- 
morphosis. 

100.  More  complex  types. — The  larger,  more  useful,  and 
usually  more  familiar  Crustacea  belong  to  the  second  divi- 
sion (subclass  Malacostraca).  It  comprises  such  animals  as 
the  shrimps,  crayfish,  lobsters,  crabs,  and  a  number  of  other 
forms  which  are  at  once  distinguished  from  the  preceding 
by  the  constant  number  of  segments  composing  the  body. 
Of  these,  five  constitute  the  head,  eight  the  thorax,  and 
seven  the  abdomen.  The  head  segments  are  always  fused 
together,  and  with  them  one  or  more  thoracic  segments 
unite  to  form  a  more  or  less  complete  cephalothorax.  Also, 


FIG.  56.— Development  of  a  barnacle 
(Lepas).    a,  larva  ;  b,  adult. 


100  ANIMAL  FORMS 

some  of  the  head  segments  give  rise  to  a  great  fold  of  the 
body-wall,  the  carapace,  which  extends  backward  and  covers 
all  or  a  part  of  the  thorax,  with  which  it  may  firmly  unite, 
as  in  the  crayfish.  The  appendages  are  usually  highly  spe- 
cialized, and  are  made  to  perform  a  variety  of  functions. 

101.  The  shrimps, — Among  the  simplest  of  these  are  the 
opossum-shrimps  (Fig.  57)  and  their  relatives,  small  trans- 


FIG.  57.— The  opossum-shrimp  (Mysis  americana). 

parent  creatures  often  seen  swimming  in  great  numbers  at 
the  surface  of  the  sea  or  hiding  among  the  seaweeds  along 
the  shore.  In  general  appearance  they  resemble  crayfishes 
or  prawns,  but  are  readily  distinguished  by  the  two-branched 
thoracic  feet.  This  "  split-foot "  character  also  occurs 
among  many  of  the  preceding  Crustacea,  and  is  generally 
a  badge  of  low  organization,  tending  to  disappear  in  the 
more  highly  organized  forms.  In  this  and  other  respects 
the  shrimps  are  especially  interesting  in  their  relation  to 
the  preceding  Crustacea,  and  in  the  fact  that  they  may 
closely  resemble  the  ancestors  of  the  modern  prawns  (Fig. 
58),  lobsters,  crayfishes,  and  crabs. 

102.  Crayfishes  and  lobsters. — The  last-mentioned  spe- 
cies and  their  allies,  usually  large  and  familiar  forms,  con- 
stitute a  group  known  as  the  decapods  (meaning  ten  feet), 
referring  to  the  number  of  thoracic  feet.  Among  the  mem- 
bers of  this  division  probably  none  are  more  familiar  than 
the  crayfishes,  which  occur  in  most  of  the  larger  rivers  and 
their  tributaries  throughout  the  United  States  and  Europe. 
It  is  their  habit  to  remain  concealed  in  crevices  of  rocks 


ARTHROPODS.      CLASS  CRUSTACEA  101 

or  in  the  mouths  of  the  burrows  which  they  excavate,  and 
from  which  they  rush  upon  the  small  fish,  the  larvae  of 


FIG.  58.—  Prawn  (Heplacarpus  brevirostris). 

many  animals,  and  other  equally  defenseless  creatures 
which  constitute  their  bill  of  fare.  In  turn  they  are 
eagerly  sought  by  certain  birds  and  four-footed  animals,  and. 
especially  in  France, 
are  extensively  used  for 
food  by  man. 

Closely  related  to 
the  crayfishes  and  dif- 
fering but  little  from 
them  structurally  are 
the  lobsters.  In  this 
country  they  are  con- 
fined to  the  rocky  coasts 
from  Xew  Jersey  to 
Labrador,  living  upon 
fish,  fresh  or  otherwise, 
various  invertebrates, 
and  occasionally  sea- 
weeds. Far  more  than 
the  crayfish,  the  lobster 
is  in  demand  as  an  arti- 
cle of  food.  By  the  aid 

Of  nets  Or  Various  traps  FIG.  59,-Tlie  crayfish  Ustacwl 


102  ANIMAL  FORMS 

millions  are  caught  each  year,  and  to  such  an  extent  has 
their  destruction  proceeded  that  in  many  places  they  are 
well-nigh  exterminated.  At  the  present  time,  however,  leg- 
islation, numerous  hatcheries,  and  a  careful  study  of  their 
life  habits  is  doing  much  to  better  matters  and  inciden- 
tally to  put  us  in  possession  of  many  interesting  zoological 
facts  along  this  line,  some  of  which  will  be  mentioned  later. 
Frequently  the  prawns,  especially  the  larger  ones,  and  a 
spiny  lobster  (Palinurus),  are  mistaken  for  crayfishes  or 
lobsters,  but  they  differ  from  them  in  the  absence  of  the 
large  grasping  claws. 

Along  almost  any  coast  some  of  these  animals  are  to  be 
found,  often  beautifully  colored  and  harmonizing  with  the 
seaweeds  among  which  they  live,  or  so  transparent  that 
their  internal  organization  may  be  distinctly  seen.  Farther 
out  at  sea  other  species  swim  in  incredible  numbers,  feed- 
ing upon  minute  organisms,  and  in  turn  fed  upon  by  numer- 
ous fishes  and  whales ;  and,  especially  on  the  Pacific  coast, 
shrimp-fishing  is  an  important  industry. 

103.  The  hermit-crabs.— The  last  of  these  long-tailed 
decapods  is  the  interesting  group  of  the  hermit-crabs, 
which  occur  in  various  situations  in  the  sea.  In  early  life 
they  take  possession  of  the  empty  shell  of  some  snail,  and 
the  protected  abdomen  becomes  soft  and  flabby,  while  the 
appendages  in  this  region  almost  completely  disappear. 
The  front  part  of  the  body,  on  the  other  hand,  continually 
grows  in  firmness  and  strength,  and  is  admirably  adapted 
for  the  continual  warfare  which  these  forms  wage  among 
themselves.  As  growth  proceeds  the  necessity  arises  for  a 
larger  shell,  and  the  crab  goes  "house-hunting"  among  the 
empty  shells  along  the  shore,  or  it  may  forcibly  extract  the 
snail  or  other  hermit  from  the  home  which  strikes  its  fancy. 

Many  of  the  hermit-crabs  enjoy  immunity  from  the 
attacks  of  their  belligerent  relatives  by  allowing  various 
hydroids  to  grow  upon  their  homes.  Others  attach  sea- 
anemones  to  their  shells  or  to  one  of  their  large  claws, 


ARTHROPODS.     CLASS  CRUSTACEA  103 

which  they  poke  into  the  face  of  any  intruder.     While 
the  anemones  or  hydroids  are  made  to  do  valiant  service 


FIG.  60. — Hermit-crab  (Pagurus  bernhardus)  in  snail  shell  covered  with  Hydractinia. 

with  their  nettle-cells,  they  also  enjoy  the  advantages  of 
a  large  food-supply  which  is  attendant  upon  the  free  ride. 

104.  The  crabs,— The  most  highly  developed  Crustacea 
are  the  crabs  or  short-tailed  decapods  which  abound  between 
tide-marks  alongshore,  and  in  diminishing  numbers  extend 
to  great  depths.  The  cephalothorax  is  usually  relatively 
wide,  often  wider  than  long,  and  the  greatly  reduced  abdo- 
men is  folded  against  the  under  side  of  the  thorax.  Corre- 
lated with  the  small  size  of  the  abdomen,  the  appendages 
of  that  region  disappear  more  or  less,  but  the  remaining 
appendages  are  similar  to  those  of  the  crayfish  or  lobsters. 
All  these  different  parts,  however,  are  variously  modified  in 
each  species  to  fit  it  for  its  own  peculiar  mode  of  life.  In 
some  forms,  such  as  the  common  cancer-crab  (Fig.  61),  the 
legs  are  comparatively  thick-set  and  possessed  of  great 
strength,  enabling  them  to  defend  themselves  against  most 
enemies.  On  the  other  hand,  there  are  the  spider-crabs 
with  small  bodies  and  relatively  long  legs,  withal  weak,  and 


104 


ANIMAL  FORMS 


yet  so  harmonizing  with  their  surroundings  that  they  are 
as  likely  to  survive  as  their  stronger  relatives.     In  this 


FIG.  61.— Kelp-crab  (Epialtus  productus)  in  upper  part  of  fignre ;  to  the  right  the 
edible  crab  ( Cancer  productus),  and  the  shore-crab  (Pugettia  richii). 

connection  it  is  interesting  to  note  that  the  giant  crab  of 
Japan,  the  largest  crustacean,  being  upward  of  twenty  feet 
from  tip  to  tip  of  the  legs,  is  a  spider-crab,  constructed  on 


FIG.  62.— The  fiddler-crab  (Gelasimus).    Photograph  by  Miss  MARY  RATHBUN. 

the  same  general  pattern  as  our   common   coast  forms. 
Between  these  two  extremes  numberless  variations  exist, 


ARTHROPODS.      CLASS  CRUSTACEA 


105 


some  for  known  reasons,  but  more  often  not  readily  under- 
stood. And  not  only  does  the  form  vary,  but  the  external 
surface  may  be  sculptured  or  beset  with  spines  or  tubercles 
which  frequently  render  the  animal  inconspicuous  amid  its 
natural  surroundings.  Such  an  effect  is  heightened  by  the 
presence  of  sponges,  hydroids,  and  various  seaweeds  which 
the  crab  often  permits  to  gather  upon  its  body. 

105.  Pill-bugs  and  sandhoppers. — Finally  there  remain  the 
groups  of  the  pill-  or  sow-bugs  (Isopods)  and  the  sand-fleas 
or  sandhoppers  (Amphipods).  In  the  first  of  these  the 
body  is  usually  small  and  compressed,  the  thorax  more  or 
less  plainly  segmented,  and  the  seven  walking  (thoracic) 
legs  are  similar.  In  the  female  each  leg  bears  at  its  base  a 
thin  membranous  plate  which  extends  inward  and  hori- 


FIG.  63.— Isopod  or  pill-bug  (Porcellio  laevis). 

zontally,  thus  forming  on  the  under  side  of  the  body  a 
brood-pouch  (Fig.  63)  in  which  the  young  develop.  As 
one  may  readily  discover  in  any  of  the  common  species, 
the  abdominal  segments  are  more  or  less  fused,  and  bear 
appendages  adapted  for  respiration  and,  in  the  aquatic 
forms,  for  swimming. 


106 


ANIMAL  FORMS 


The  marine  isopods  occur  in  the  sand,  under  rocks,  and 
in  the  seaweeds ;  many  are  parasitic  upon  fishes ;  and  the  ter- 
restrial forms  (Fig.  63)  are  very  common  objects  under  old 


FIG.  64.— Amphipods  or  sand-fleas  (Gammarus,  upper  species,  and  Capretta). 

logs  and  in  cellars,  where  they  live  chiefly  on  vegetable  mat- 
ter. In  the  sand-fleas  the  body  is  compressed  from  side  to 
side,  and  while  the  thorax  shows  distinct  segments,  the  legs 
are  frequently  dissimilar,  and  some  may  bear  pincers.  One 
of  their  most  distinctive  marks  concerns  the  last  three  ab- 
dominal appendages,  which  are  usually  modified  for  leaping. 
The  sand-fleas  (Fig.  64)  are  familiar  objects  to  any  one 
who  has  collected  along  the  beach  and  has  turned  over  the 
cast-up  seaweeds,  while  numbers  of  small  species  often  oc- 
cur among  the  plants  in  our  fresh-water  ponds.  Some  most 
curious  and  highly  modified  forms,  whose  general  appear- 
ance is  shown  in  the  lower  part  of  Fig.  64,  occur  among 


ARTHROPODS.      CLASS  CRUSTACEA  107 

hydroid  colonies,  with  which  their  bodies  harmonize  in 
form  and  color.  And,  lastly,  most  bizarre  creatures,  known 
as  "  whale-lice,"  attach  themselves  to  the  skin  of  whales,  of 
which  each  species  acts  as  host  for  one  or  more  kinds. 

106.  Internal  organization.— Most  Crustacea  are  carnivo- 
rous, preying  upon  almost  any  of  the  smaller  animals  within 
convenient  reach ;  a  much  smaller  number  live  on  vege- 
table food ;  and  there  are  many,  such  as  the  crayfishes,  lob- 
sters, and  numerous  crabs,  which  are  also  notorious  scaven- 
gers. In  these  latter  forms  the  food  is  held  in  one  of  the 
large  pincers,  torn  into  shreds  by  the  other,  and  transferred 
to  the  mouth-parts,  where,  as  in  all  Crustacea,  it  is  soon 
reduced  to  a  pulp  by  their  rapid  movements.  In  many 
species  the  food  is  now  ready  for  the  digestive  process, 
but  not  so  in  the  higher  forms.  If  the  stomach  of  any  of 
these,  for  example,  the  crabs  or  crayfishes,  be  opened,  three 
(Fig.  65,  s)  large  teeth  operated  by  powerful  muscles  will 
be  noted,  and  beyond  these  a  strainer  consisting  of  many 
closely  set  hairs.  In  operation  this  "  gastric  mill "  takes 
the  food  passed  on  from  the  mouth-parts,  and  crushes  and 
tears  it  until  fine  enough  to  pass  through  the  strainer, 
whereupon  it  is  dissolved  by  the  juices  from  the  liver  and 
is  absorbed  as  it  passes  down  the  intestine. 

The  circulatory  system  is  usually  highly  developed,  and 
consists  of  a  heart,  in  some  species  almost  as  long  as  the 
body,  though  usually  shorter  (Fig.  65),  from  which  two  or 
more  arteries  branch  to  all  parts  of  the  body.  Here  the 
blood,  instead  of  emptying  into  definite  veins,  pours  into  a 
series  of  spaces  or  sinuses  in  among  the  muscles  and  other 
organs  of  the  body,  through  which  it  makes  its  way  back  to 
the  heart.  During  this  return  journey  it  is  usually  made 
to  traverse  definite  respiratory  organs,  either  situated  upon 
the  legs  or,  as  feathery  outgrowths,  upon  the  sides  of  the 
body,  and  generally  concealed  under  the  carapace.  A  por- 
tion of  the  blood  is  also  continually  sent  to  the  kidneys, 
which  are  located  either  at  the  base  of  the  second  antennae 


108  ANIMAL  FORMS 

(and  known  as  green  glands),  as  in  the  crayfishes  or  crabs, 
or  on  the  second   maxillae  (shell-glands)  in  many  of  the 


PIG.  65.— Dissection  of  crayfish,    b,  brain  ;  k,  heart ;  i,  intestine  ;  k,  kidney  ;  /,  liver  ; 
n,  nerve-cord  ;  r,  reproductive  organ  ;  s,  stomach,  showing  two  teeth  in  position. 

simpler  crustaceans.  Their  method  of  operation  is  much 
like  that  of  the  kidneys  in  the  earthworm. 

107.  Nervous  system  and  special  senses. — The  nervous  sys- 
tem also  shows  a  decided  resemblance  to  that  of  the  anne- 
lids. The  cerebral  ganglia  or  brain  is  situated  above  the 
alimentary  canal  in  the  head,  and  connects  with  the  ven- 
trally  lying  cord  by  a  collar.  As  in  the  earthworm,  this 
ventral  cord  is  double,  and  bears  a  pair  of  swellings  or  gan- 
glia in  each  segment.  In  the  crayfish,  crabs,  and  other 
highly  modified  forms,  where  the  segments  tend  to  fuse, 
several  of  these  ganglia  may  also  unite,  and  except  in  early 
life  their  number  cannot  be  determined. 

Among  the  less  specialized  Crustacea  the  order  of  intel- 
ligence is  low,  though  perhaps  it  may  prove  to  be  higher 
than  is  usually  supposed  when  such  forms  have  been  more 
thoroughly  studied.  The  following  quotation  relating  to 
the  lobster  applies  even  more  to  the  higher  forms,  the 
crabs  :  "  Sluggish  as  it  often  appears  when  out  of  water  and 
when  partially  exhausted,  it  is  quite  a  different  animal  when 
free  to  move  at  will  in  its  natural  environment  on  the  sea- 


ARTHROPODS.      CLASS  CRUSTACEA  109 

bottom.  It  is  very  cautious  and  cunning,  capturing  its 
prey  by  stealth,  and  with  weapons  which  it  knows  how  to 
conceal.  Lying  hidden  in  a  bunch  of  seaweed,  in  a  crevice 
among  the  rocks,  or  in  its  burrow  in  the  mud,  it  waits  until 
its  victim  is  within  reach  of  its  claws,  before  striking  the 
fatal  blow.  The  senses  of  sight  and  hearing  are  probably 
far  from  acute,  but  it  possesses  a  keen  sense  of  touch  and 
of  smell,  and  probably  also  a  sense  of  taste." 

Although  enclosed  in  a  horny  and  often  very  thick  and 
strong  armor,  the  sense  of  touch  is  very  keen  in  the 
Crustacea  and  in  arthropods  generally.  On  many  of  the 
more  exposed  portions  delicate  hairs  or  pits  connected 
with  the  nervous  system  occur  in  great  abundance.  Some 
of  these,  usually  on  the  antennae,  undoubtedly  serve  in 
detecting  odors,  but  the  remainder  are  considered  to  be 
tactile.  In  the  higher  Crustacea,  such  as  the  crayfish, 
lobsters,  and  crabs,  ears  are  usually  found,  consisting  of 
sacs  lined  with  similar  delicate  hairs,  and  containing  sev- 
eral minute  grains  of  sand,  which  in  many  cases  make  their 
way  through  the  small  external  opening.  Vibrations  com- 
ing through  the  water  gently  shake  the  grains  of  sand, 
causing  them  to  strike  against  the  hairs  which  communi- 
cate with  the  nervous  system — a  very  simple  ear,  yet  suffi- 
cient for  the  needs  of  the  animals. 

The  eyes  of  the  Crustacea  and  arthropods  in  general  are 
either  simple  or  compound.  The  simple  and  frequently 
single  eyes  usually  consist  of  a  relatively  few  cells  embedded 
in  a  quantity  of  pigment  and  connected  with  the  nervous 
system.  It  is  doubtful  whether  they  perceive  objects  as 
anything  more  than  highly  blurred  images,  and  perhaps 
they  merely  recognize  the  difference  between  light  and 
darkness.  The  compound  eyes,  on  the  other  hand,  are 
remarkably  complex  structures,  often  borne  on  the  tops  of 
movable  stalks,  as  in  the  common  crabs  and  crayfishes. 
Each  consists  of  an  external  transparent  cornea,  divided 
into  numerous  minute  hexagonal  areas  corresponding  to  as 


110  ANIMAL   FORMS 

many  internal  rods  of  cells,  provided  with  an  abundant 
nerve-supply.  These  latter  elements  may  perhaps  repre- 
sent simple  eyes  grouped  together  to  form  the  compound 
one ;  and  it  appears  possible  that  each  element  may  form 
a  complete  image  of  an  object,  as  each  of  our  eyes  is  known 
to  do.  On  the  other  hand,  many  hold  that  the  complete 
eye  forms  only  one  image,  a  mosaic,  each  element  con- 
tributing its  share. 

108.  Growth  and  development. — As  we  have  seen,  the 
simpler  Crustacea  hatch  as  minute  larvae  (Fig.  56),  and  dur- 
ing their  growth  to  the  adult  condition  are  especially  sub- 
ject to  the  attacks  of  multitudes  of  hungry  enemies.  In 
the  higher  forms,  such  as  the  crabs,  some  of  these  early 
transformations  take  place  while  the  young  are  still  within 
the  egg  and  attached  to  the  parent.  Accordingly,  the  little 
ones  are  fairly  similar  to  their  parents,  and  their  later  his- 
tory is  very  well  exemplified  by  the  lobster. 

The  eggs  of  the  lobster  are  most  frequently  hatched  in 
the  summer  months,  usually  July,  after  they  have  been 
carried  by  the  parent  for  upward  of  a  year.  The  young, 
about  a  third  of  an  inch  in  length,  at  once  disperse,  undergo 
four  or  five  moults  during  the  next  month,  then,  ceasing 
their  swimming  habits,  settle  to  the  bottom  among  the 
rocks.  At  this  time,  twice  their  original  size,  they  closely 
resemble  their  parents,  and  their  further  development  is 
largely  an  increase  in  size.  "  The  growth  of  the  lobster, 
and  of  every  arthropod,  apparently  takes  place,  from  in- 
fancy to  old  age,  by  a  series  of  stages  characterized  by  the 
growth  of  a  new  shell  under  the  old,  by  the  shedding  of 
the  outgrown  old  shell,  a  sudden  increase  in  size,  and  the 
gradual  hardening  of  the  shell  newly  formed.  Not  only  is 
the  external  skeleton  cast  off  in  the  moult  and  the  linings 
of  the  masticatory  stomach,  the  esophagus,  and  intestine, 
but  also  the  internal  skeleton,  which  consists  for  the  most 
part  of  a  complicated  linkwork  of  hard  tendons  to  which 
muscles  are  attached." 


ARTHROPODS.     CLASS  CRUSTACEA 


111 


109.  Peripatus  (class  Onychophora).—  It  is  generally  be- 
lieved that  the  Crustacea,  insects,  and  spiders,  together 
with  their  numerous  relatives,  trace  their  ancestry  back  to 
animals  that  bore  a  certain  resemblance  to  the  segmented 
worms.     Most  of  these  ancient  types  have 

long  been  extinct,  but  here  and  there 
throughout  the  earth  we  occasionally  meet 
with  them. 

Among  the  most  interesting  of  these 
are  a  few  widely  distributed  species  belong- 
ing to  the  genus  Peripatus  (Fig.  66),  but  as 
they  are  comparatively  rare  we  shall  dis- 
miss them  with  a  very  brief  description. 
They  usually  dwell  in  warm  countries,  un- 
der rocks  and  decaying  wood,  emerging  at 
night  to  feed  on  insects,  which  they  ensnare 
in  the  slime  thrown  out  from  the  under 
surface  of  the  head.  Their  external  form, 
their  excretory  system,  and  various  other 
organs  are  worm-like.  On  the  other  hand, 
the  appendages  are  jointed,  and  one  pair 
has  been  modified  into  jaws.  The  peculiar 
breathing  organs  characteristic  of  the  in- 
sects are  also  present.  Peripatus  therefore 
gives  us  an  interesting  link  between  the 
worms  and  insects,  and  also  affords  an  idea 
of  the  primitive  insects  from  which  the 
modern  forms  have  descended. 

110.  The  centipeds  and  millipeds  (class 
Myriapoda). — Many  of  the  myriapods — that 
is,  the  centipeds  and  thousand-legged  worms 
— are  familiar  objects  under  logs  and  stones 
throughout  the  United  States.     The  first  of  these  (Fig.  67) 
are  active,  savage  creatures,  devouring  numbers  of  small 
animals,  which  they  sting  by  means  of  poison-spines  on  the 
tips  of  the  first  pair  of  legs.    The  bite  of  the  larger  tropical 

30 


PIG.  66.— Peripatus 
(Peripatus  eiseni). 
Twice  the  natural 
size. 


112 


ANIMAL  FORMS 


species  especially  causes  painful  but  not  fatal  wounds  in 
man. 

On  the  other  hand,  the  millipeds  (Fig.  68)  or  thousand- 
legs  are  cylindrical,  slow-going  animals,  feeding  on  vegetable 


PIG.  67.— Centiped. 
One-half  natural  size. 


FIG.  68.— Thousand-legs  or  milliped  (Julus). 
Natural  size. 


substances  without  causing  any  particular  damage,  except 
in  the  case  of  certain  species,  which  work  injury  to  crops. 
When  disturbed  they  make  little  effort  to  escape,  but  roll 
into  a  coil  and  emit  an  offensive-smelling  fluid,  which  ren- 
ders them  unpalatable  to  their  enemies. 

All  present  a  great  resemblance  to  the  segmented  worms, 
as  their  popular  names  often  testify;  but,  on  the  other 
hand,  many  points  in  their  organization  indicate  a  closer 
relationship  to  the  insects.  As  in  the  latter,  the  head  is 
distinct,  and  bears  a  pair  of  antennas,  the  eyes,  and  two  or 
three  pairs  of  mouth-parts.  The  trunk  is  more  worm-like, 
and  consists  of  a  number  of  similar  segments,  each  bearing 


ARTHROPODS.     CLASS  CRUSTACEA  113 

one  or  two  pairs  of  jointed  legs.  In  their  internal  organ- 
ization the  character  of  the  various  systems  closely  resem- 
bles that  of  the  insects,  and  will  be  more  conveniently 
described  in  that  connection. 

Among  the  myriapods  the  females  are  usually  larger 
than  the  males.  Some  of  the  centipeds  deposit  a  little 
mass  of  eggs  in  cavities  in  the  earth  and  then  abandon 
them,  while  others  wrap  their  bodies  about  them  and  pro- 
tect them  until  the  young  are  hatched.  The  millipeds  lay 
in  the  same  situations,  but  usually  plaster  each  egg  over 
with  a  protective  layer  of  mud.  After  several  weeks  the 
young  appear,  often  like  their  parents  in  miniature,  but  in 
other  species  quite  unlike,  and  requiring  several  molts  to 
complete  the  resemblance. 


OF  THE 

UNIVERSITY 

OF 


|^.  CHAPTER  X 

ARTHROPODS  (Continued).    CLASS  INSECTS 

111.  Their  numbers. — It  has  been  estimated  that  upward 
of  three  hundred  thousand  named  species  of  insects  are 
known  to  the  zoologist,  and  that  these  represent  a  fifth,  or 
possibly  a  tenth,  of  those  living  throughout  the  world.   Many 
of  these  species,  as  the  may-flies  and  locusts,  are  represented 
by  millions  of  individuals,  which  sometimes  travel  in  such 
great  swarms  that  they  darken  the  sky.     With  nearly  all 
of  these  the  struggle  for  existence  is  fierce  and  unrelenting, 
and  it  is  little  wonder   that  such   plastic  animals  have 
changed  in  past  times  and  are  now  becoming  modified  in 
order  to  adapt  themselves  to  new  situations  where  food  is 
more  abundant  and  the  conditions  less  severe.     Owing  to 
such  modifications  we  find  some  species  fitted  for  flying, 
others  for  running  and  leaping,  or  for  a  life  underground, 
and  many  for  a  part  or  all  of  their  lives  are  aquatic  in  their 
habits. 

112.  External   features.— The  body  of  an   insect— the 
grasshopper,  for  example — consists  of  a  number  of  rings 
arranged  end  to  end,  as  we  have  seen  them  in  the  Crustacea 
and  the   segmented  worms.      In  the  abdomen  these  are 
clearly  distinct,  but  in  the  thorax,  and  especially  the  head, 
they  have  become  so  intimately  united  that  their  number 
is  a  matter  of  uncertainty.      These  three  regions— head, 
thorax,  and  abdomen — are  usually  clearly  defined  in  most 
insects,  but  they  are  modified  in  innumerable  ways  in  ac- 
cordance with  the  animal's  mode  of  life. 

114 


ARTHROPODS.    CLASS  INSECTS  115 

The  head  usually  carries  the  eyes,  a  pair  of  feelers  (an- 
tennae), and  three  pairs  of  mouth-parts  which  may  be  fash- 
ioned into  a  long,  slender  tube  to  be  used  in  sucking,  and 
frequently  as  a  piercing  organ  ;  or  they  may  be  constructed 
for  cutting  and  biting.  The  thorax  bears  three  pairs  of 
legs  and  often  one  or  two  pairs  of  wings.  The  appendages 
of  the  abdomen  are  usually  small  and  few  in  number,  or 
even  absent. 

113.  Internal  anatomy.— The  restless  activity  of  insects 
is  proverbial.  Some  appear  to  be  incessantly  moving  about, 
either  on  the  wing  or  afoot,  and  are  endowed  with  com- 
paratively great  strength.  Ants  and  beetles  lift  many  times 
their  own  weight.  Numerous  insects  are  able  to  leap  many 
times  their  own  length,  and  others  perform  different  kinds 
of  work  with  a  vigor  and  rapidity  unsurpassed  by  any  other 
class  of  animals.  As  is  to  be  expected,  the  muscular  sys- 
tem is  well  developed,  and  exhibits  a  surprising  degree  of 
complexity.  Over  five  hundred  muscles  are  required  for 
the  various  movements  of  our  own  bodies,  but  in  some  of 
the  insects  more  than  seven  times  this  number  exist.  The 
amount  of  food  necessary  to  supply  this  relatively  immense 
system  with  the  required  nourishment  is  correspondingly 
large.  Many  insects,  especially  in  an  immature  or  larval 
condition,  devour  several  times  their  own  weight  each  day. 
Their  food  may  consist  of  the  juices  of  animals  or  plants, 
which  they  suck  out,  or  of  the  firmer  tissues,  which  are 
bitten  or  gnawed  off. 

Not  only  do  the  mouth-parts  stand  in  direct  relation  to 
the  habits  of  the  animal  and  to  its  food,  but,  as  we  have 
often  noticed  before,  the  internal  organization  is  also 
adapted  for  the  digestion  and  distribution  of  the  nutritive 
substances  in  the  most  economical  way.  For  this  reason 
we  find  the  alimentary  canal  differing  widely  in  the  various 
forms  of  insects.  In  each  case  it  extends  from  the  mouth 
to  the  opposite  end  of  the  animal,  and  ordinarily  consists 
of  a  number  of  different  parts.  In  the  insect  shown  in 


116 


ANIMAL  FORMS 


Fig.  69  the  mouth  soon  leads  into  the  esophagus,  which 
in  turn  leads  into  the  crop  that  serves  to  store  up  the  food 
until  ready  for  its  entry  into  the  stomach  ;  or  in  some  of 
the  ants,  bees,  and  wasps  it  may  contain  material  which 

may  be  disgorged  and  fed 
to  the  young.  In  many 
cases  the  stomach  is  small 
and  ill-defined  as  in  Fig.  69, 
and  again  it  may  reach 
enormous  dimensions,  near- 
ly  filling  the  body.  It  may 
also  bear  numerous  lobes  or 
delicate  hair-like  processes, 
which  afford  a  greater  sur- 
face for  the  absorption  of 
food.  Behind  the  stomach 
are  a  number  of  slender 
outgrowths  that  are  believed 
to  act  as  kidneys.  Beyond 
their  insertion  lies  the  in- 
testine, which,  like  the 
stomach,  is  the  subject  of 
many  modifications  in  the 

•,.«  ,    -,  •      -, 

different  kinds  of  insects. 

The  digested  food  is  rap- 

idly absorbed  through  the  coats  of  the  stomach  and  intes- 
tine and  enters  a  circulatory  system  which  reminds  us  of 
what  exists  in  many  of  the  Crustacea.  The  heart  is  situ- 
ated above  the  digestive  tract,  and  from  it  arteries  pass  out 
to  different  parts  of  the  body.  Here  the  blood  leaves  the 
vessels  and  is  poured  directly  into  the  spaces  among  the 
viscera,  whence  it  is  finally  conducted  through  irregular 
channels  to  the  heart  by  its  pulsations. 

In  the  Crustacea  the  blood  is  made  to  pass  through  a 
respiratory  system  usually  in  the  form  of  definite  gills,  and 
the  oxygen  with  which  it  is  charged  is  distributed  to  all 


FIG.  69.-Cockroach,  dissected  to  show  aii- 

mentary  canal,  al.  c.—  After  HATSCHEK 


ARTHROPODS.    CLASS  INSECTS  117 

parts  of  the  body.  In  the  insects  the  blood  serves  almost 
entirely  to  carry  the  food,  and  the  oxygen  is  conveyed 
through  the  animal  by  a  remarkable  contrivance  found 
only  in  the  insects,  the  spiders,  and  a  few  related  forms. 

114.  Respiratory  system. — If  we  examine  an  insect,  the 
grasshopper  for  example,  we  find  a  number  of  small  brown 
spots  on  each  side  of  the  abdomen,  each  of  which  under  a 
magnifying-glass  is  seen  to  be  perforated  by  a  narrow  slit. 
Carefully  opening  the  body,  we  find  that  each  slit  is  in 
communication  with  a  white,  glistening  tube  that  rapidly 
branches  and  penetrates  to  all  parts  of  the  animal.     When 
the  body  is  expanded  the  air  rushes  into  the  outer  openings, 
on  through  the  open  tubes,  and  is  distributed  with  great 
rapidity  to  all  the  tissues  of  the  body.     In  many  insects 
some  of  these  tubes  connect  with  air-sacs  which  probably 
serve   to  buoy  up   the   insect   during  its  flights   through 
the  air. 

115.  Wingless  insects  (Thysanura). — The  simplest  of  all 
insects  are  the  fishmoths  and  springtails,  relatively  small 
organisms  covered  with  shining  scales  or  hairs.     The  first 
of  these  is  occasionally  seen  running  about  in  houses  feed- 
ing upon  cloth  and  other  substances,  while  the  latter  live 
in  damp  places  under  stones  and  logs.     They  are  without 
wings,  but  are  able  to  run  rapidly  and  to  leap  considerable 
distances.     In  addition  to  the  ordinary  appendages,  the 
abdomen  bears  what  are  perhaps  rudimentary  legs,  a  fact 
which,    together   with   their   relatively   simple   structure, 
strengthens   the   belief  that   the  insects  have   descended 
from  centiped-like  ancestors. 

116.  Grasshoppers,  crickets,  katydids,  etc.  (Orthoptera). — 
Eising  higher  in  the  scale  of  insect  life,  we  arrive  at  the  group 
of  the   cockroaches,    crickets,   grasshoppers,   locusts,  and 
other  related  insects.     Four  wings  are  present,  the  first  pair 
thickened  and  overlapping  the  second  thinner  pair.     The 
latter  are  folded  lengthwise  like  a  fan,  which  is  said  to  have 
given  the  name  Orthoptera  (meaning  straight-winged)  to 


118  ANIMAL  FORMS 

this  group  of  insects.  These  extend  all  over  the  world, 
being  particularly  abundant  in  the  warmer  countries,  and 
their  strong  biting  mouth-parts  and  voracious  appetites 
render  many  of  them  dreaded  pests  to  the  farmer.  The 
cockroaches  are  nocturnal  in  their  habits,  racing  about  at 
night,  devouring  victuals  in  the  pantry  and  gnawing  the 
bindings  of  books.  During  the  day  their  flat  bodies  enable 
them  to  secrete  themselves  in  crevices  wherever  there  is 
sufficient  moisture. 

In  the  grasshoppers,  locusts,  katydids,  and  crickets  the 
body  is  more  cylindrical,  and  the  hind  pair  of  legs  are  often 
greatly  lengthened  for  leaping.  The  crickets  and  katydids 

are  nocturnal,  the  former  re- 
maining by  day  in  burrows 
which  they  construct  in  the 
earth,  the  latter  resting  qui- 
etly in  the  trees.  At  night 

PIG.  70.— The  Rocky  Mountain  locust.—         ,  „  ** 

After  RILEY,  from  The  Insect  World.         tnej    ±east     uP<>n    Vegetable 

matter  principally,  though 

some  species  are  known  to  prey  on  small  animals.  Those 
insects  we  usually  term  grasshoppers  (properly  called  lo- 
custs) are  specially  destructive  to  vegetation.  Some  spe- 
cies are  strong  fliers,  and  this,  connected  with  their  abil- 
ity to  multiply  rapidly,  renders  them  greatly  dreaded  pests. 
They  have  been  described  as  flying  in  great  swarms,  form- 
ing black  clouds,  even  hiding  the  sun  as  far  as  the  eye 
could  reach.  The  noise  made  by  their  wings  resembled 
the  roar  of  a  torrent,  and  when  they  settled  upon  the  earth 
every  vestige  of  leaf  and  delicate  twig  soon  disappeared. 

The  eggs  of  the  majority  of  Orthoptera  are  laid  in  the 
ground,  where  they  frequently  remain  through  the  winter. 
When  hatched  the  young  quite  closely  resemble  the  parents, 
and,  after  a  relatively  slight  metamorphosis,  assume  the 
adult  form. 

117.  Dragon-flies,  may-flies,  white  ants,  etc.  (Pseudoneurop- 
tera). — The  dragon-flies,  caddis-flies,  may-flies,  and  white  ants 


ARTHROPODS.    CLASS  INSECTS 


119 


possess  four  thin  and  membranous  wings  incapable  of  being 
folded.  These  possess  a  network  of  delicate  nervures,  giv- 
ing the  name  Neuroptera  (meaning  nerve-winged)  to  the 
class.  Of  the  forms  mentioned  above,  all  but  the  white 
ants  lay  their  eggs  in  the  water,  and  the  developing  larvse 


FIG.  71.-Dragon-fly  (Li bdlula  pulchelld). 

spend  their  lives  in  this  medium  until  the  time  comes  for  their 
complete  metamorphosis  into  the  adult.  The  larvae  of  the 
caddis-flies  protect  themselves  within  a  tube  of  stones  or  sticks 
bound  together  with  silken  threads,  which  they  usually 
attach  to  the  under  side  of  stones  in  running  water.  On 
the  other  hand,  the  young  of  the  dragon-  and  may-flies,  pro- 
vided with  strong  jaws,  are  active  in  the  search  of  food  and 
very  voracious.  In  time  they  emerge  from  their  larval  skin 
and  the  water  in  which  they  live,  and  after  a  life  spent  on 
the  wing  they  deposit  their  eggs  and  perish.  The  adult 
ant-lion,  a  type  of  the  related  order  (Neuroptera),  which 
has  somewhat  the  appearance  of  a  small  dragon-fly,  lays  its 
eggs  in  light  sandy  soil.  In  this  the  resulting  larvae  exca- 
vate funnel-shaped  pits,  at  the  bottom  of  which  they  lie  con- 


120 


ANIMAL  FORMS 


cealed.  Insects  stumbling  into  their  pitfalls  are  pelted  with 
sand,  which  the  ant-lion  throws  at  them  with  a  jerky  motion 
of  the  head,  and  are  speedily  tumbled  down  the  shifting 
sides  of  the  funnel  to  be  seized  and  devoured. 

While  the  white  ants  are  not  in  any  way  related  to  the 
true  ants,  they  possess  many  similar  habits.  Associated  in 
great  companies,  they  excavate  winding  galleries  in  old  logs 
and  stumps,  and,  further,  are  most  interesting  because  of 
the  division  of  labor  among  the  various  members.  The 
wingless  forms  are  divided  inta  the  workers,  which  exca- 
vate, care  for  the  young,  and  otherwise  labor  for  the  good 
of  the  others;  and  into  the  soldiers,  huge-headed  forms, 


FIG.  72.— Ant-lion  larva  plowing  its  way  through  the  sand  (upper  figure)  while  an- 
other is  commencing  the  excavation  of  a  funnel-shaped  pit  similar  to  one  on  right. 
Photograph  by  A.  L.  MELANDER  and  C.  T.  BRUES. 

whose  strong  jaws  serve  to  protect  the  colony.  The  re- 
maining winged  forms  are  the  kings  and  queens.  In  the 
spring  many  of  the  royalty  fly  away  from  home,  shed  their 
wings,  unite  in  pairs,  and  set  about  to  organize  a  colony. 
The  queen  rapidly  commences  to  develop  eggs,  and  in  some 


ARTHROPODS.    CLASS  INSECTS 


121 


species  her  body  becomes  so  enormously  distended  with 
these  that  she  loses  the  power  of  locomotion  and  requires 
to  be  fed.  A  single  queen  has  been  known  to  lay  eggs  at 
the  rate  of  sixty  per  minute  (eighty  thousand  a  day),  and 


FIG.  73.— Termites  or  white  ants,     a,  queen  ;  b,  winged  male  ;  c,  worker ;  d,  soldier. 

those  destined  to  royal  rank  are  so  nursed  that  they  advance 
farther  in  their  development  than  the  remaining  sterile 
and  wingless  forms. 

118.  The  bugs  (Hemiptera).— The  large  and  varied  group 
of  the  bugs  (Hemiptera)  includes  a  number  of  semi-aquatic 
species,  such  as  the  water-boatmen,  often  seen  rowing 
themselves  along  in  the  ponds  by  means  of  a  pair  of  oar- 
shaped  legs,  in  search  of  other  insects.  Somewhat  similar 
at  first  sight  are  the  back-swimmers,  with  like  rowing 
habits,  but  unique  in  swimming  back  downward.  Both  of 
these  bugs  frequently  float  at  the  surface,  and  when  about 
to  undertake  a  subaquatic  journey  they  may  be  seen  to 
imprison  a  bubble  of  air  to  take  along.  Closely  related  are 
the  giant  water-bugs  (Fig.  74),  which  often  fly  from  pond 
to  pond  at  night.  In  such  flights  they  are  frequently 


122 


ANIMAL  FORMS 


attracted  by  lights,  and  have  come  to  be  called  "  electric- 
light  bugs." 

Among  our  most  dreaded  insect  pests  are  the  chinch- 
bugs—small  black-and-white  insects,  but  traveling  in  com- 
panies aggregating  many  millions. 
As  they  go  they  feed  upon  the 
stems  and  leaves  of  grain,  which 
they  devour  with  extraordinary  ra- 
pidity. The  squash-bug  family  is 
also  extensive,  and  destructive  to 
the  young  squash  and  pumpkin 
plants  in  the  early  spring. 

The  lice  are  small,  curiously 
shaped  bugs,  which  suck  the  blood 
of  other  animals.  The  plant-lice, 
also  small,  suck  the  juices  of 
plants,  and  are  often  exceedingly 
destructive.  This  is  especially  true 
of  the  phylloxera,  a  plant-louse 
which  causes  annually  the  loss  of 
millions  of  dollars  among  the  vine- 
yards of  this  and  other  countries. 

Even  more  destructive  are  the  scale-insects,  curiously  mod- 
ified forms,  of  which  the  wingless  females  may  be  found  on 
almost  any  fruit-tree  and  on  the  plants  in  conservatories, 
their  bodies  covered  with  a  downy,  waxy,  or  other  kind  of 
covering,  beneath  which  they  remain  and  lay  their  eggs, 

119.  The  flies  (Diptera). — The  group  of  the  Diptera 
(meaning  two-winged)  includes  the  gnats,  mosquitoes,  fleas, 
house-flies,  horse-flies  (Fig.  75),  and  a  vast  company  of 
related  forms.  Only  a  single  pair  of  wings  is  present,  the 
second  pair  being  rudimentary  or  fashioned  into  short, 
thread-like  appendages  known  as  balancers,  though  they 
probably  act  as  sensory  organs  and  are  not  directly  con- 
cerned with  flight.  The  mouth-parts  are  adapted  for  pier- 
cing and  sucking.  The  eyes,  constructed  on  the  same  plan 


FIG.  74.— Giant  water-bug  (Ser- 
phus  dilatatus),  with  eggs  at- 
tached. 


ARTHROPODS.    CLASS  INSECTS 


123 


FIG.  75.— Horse-fly  (T7, 


as  those  of  the  Crustacea,  are  comparatively  large,  and  are 
frequently  composed  of  a  great  number  of  simple  eyes 
united  together,  upward  of  four 
thousand  forming  the  eye  of  the 
common  house-fly. 

These  insects  are  widely  distrib- 
uted throughout  the  world,  where 
they  inhabit  woods,  fields,  or  houses 
as  best  suits  their  needs.  Their 
food  is  varied.  Some  suck  the 
juices  of  plants,  others  attack  ani- 
mals, and,  while  many  are  trouble- 
some pests,  others,  especially  in  the 
early  stages  of  their  existence,  are 
of  great  benefit. 

120.  Familiar  examples. — Owing 
to  the  widely  different  habits  and 
structure  of  the  members  of  this  group,  we  shall  briefly 
consider  two  examples,  the  mosquito  and  the  house-fly, 
which  will  give  us  a  fairly  good  idea  of  the  characteristics 
of  all.  The  eggs  of  the  mosquito  are  laid  in  sooty-look- 
ing masses  on  the  surface  of  stagnant  pools.  Within  a 
very  short  time  the  young  hatch,  and,  owing  to  their  pecul- 
iar swimming  movements,  are  known  as  "  wrigglers."  They 
are  then  active  scavengers,  devouring  vast  quantities  of 
noxious  substances  and  performing  a  valued  service.  They 
frequently  rise  to  the  surface,  take  air  into  the  tracheal 
system,  which  opens  at  the  posterior  end  of  the  body,  and 
descend  again.  After  an  increase  in  growth  and  many  in- 
ternal changes  resulting  in  a  chrysalis-like  stage,  they  rise 
to  the  surface,  split  the  shell,  and,  using  the  latter  as  a  float, 
carefully  balance  themselves  and  soon  fly  away. 

The  house-fly  usually  lays  its  eggs  in  decaying  vegetable 
matter,  and  the  young,  maggot-like  in  form,  are  active 
scavengers.  They  too  undergo  deep-seated  changes  during 
the  next  few  days,  finally  transforming  into  the  adult. 


124 


ANIMAL  FORMS 


Many  of  this  great  group  of  the  flies  spend  their  early  life 
in  the  water  or  other  medium  acting  as  scavengers ;  but,  on 
the  other  hand,  numbers  attack  domestic  and  other  animals, 
and  throughout  their  entire  lives  are  an  intolerable  plague. 
121.  The  beetles  (Coleoptera).— Owing  to  the  ease  of  pres- 
ervation and  their  bright  colors,  the  beetles  have  probably 
been  more  widely  collected  than  other  insects.  Fully  ten 


FIG.  76.— Long-horned  borer  (Ergateti).     Larva  (left-hand  figure),  pupa,  and  adult 

insect. 

thousand  distinct  species  are  known  in  North  America 
alone.  They  are  all  readily  recognized  by  the  two  firm, 
horny  sheaths  enclosing  the  two  membranous  wings,  which 
alone  are  organs  of  flight.  The  mouth  is  provided  with 
jaws,  which  are  used  in  gnawing.  Some  prey  on  noxious 
insects  or  upon  decaying  vegetable  or  animal  matter,  and 
are  often  highly  beneficial ;  but  others  attack  our  trees  and 
domestic  animals,  and  work  incalculable  damage. 


ARTHROPODS.    CLASS  INSECTS  125 

In  some  of  the  stag-  or  wood-beetles  (Fig.  76),  which 
we  may  select  as  types,  the  adults  are  often  found  crawling 
about  on  or  beneath  the  bark  of  trees,  living  on  sap  or 
small  animals.  The  eggs  laid  in  these  situations  develop 
into  grub-like  larvae,  which  bore  their  way  through  living 
or  dead  wood,  and  in  this  condition  sometimes  live  four  or 
five  years.  They  then  transform  into  quiescent  pupae  (Fig. 
76),  which  finally  burst  their  shells  and  emerge  in  the 
adult  form.  Others,  like  water-beetles  and  the  whirligig- 
beetles,  whose  mazy  motions  are  often  seen  on  the  surface 
of  quiet  streams,  pass  the  larval  period  in  the  water. 
Under  somewhat  different  conditions  we  find  the  potato- 
bugs,  lady-bugs,  fire-flies,  and  their  innumerable  relatives, 
but  the  changes  they  undergo  in  becoming  adult  are  essen- 
tially the  same  as  those  described  for  the  other  members  of 
the  order. 

122.  The  moths  and  butterflies  (Lepidoptera). — The  moths 
and  butterflies  occur  all  over  the  world.  In  their  mature 


FIG.  7?.— Monarch-butterfly  (Anosia  plexippus}.    From  photograph  by  A.  L.  MELAN- 
DER  and  C.  T.  BRUES. 

state  they  are  possessed  of  a  grace  of  form  and  movement 
and  a  brilliancy  of  coloration  that  elicit  our  highest  admi- 
ration. The  mouth-parts  are  developed  into  a  long  pro- 
boscis, which  may  be  unrolled  and  used  to  suck  the  nectar 
out  of  flowers,  though  in  many  of  the  adult  moths,  which 
never  feed,  it  may  remain  unused.  The  wings,  four  in 
number,  are  covered  with  beautiful  overlapping  scales  that 


126 


ANIMAL   FORMS 


adhere  to  our  fingers  when  handled.     This  feature,  and 
the  general  plan  of  the  body,  which  is  much  the  same 


•:».         «i  *w  * 

FIG.  ?8.--The  silver-spot  (Argynnis  cybele).    Photograph  by  A.  L.  MELANDER  and 

C.  T.  BRUES. 

throughout  the  group,  enables  us  to  recognize  most  of 
them  at  once. 

123.  Development  and  metamorphosis. — In  some  of  the 
simplest  insects,  as  in  the  bugs,  the  young  at  birth  resemble 
their  parents.  In  other  insects  the  resemblance  is  not  so 
close.  The  young  grasshopper,  for  example,  hatches,  from 
an  egg  laid  in  the  ground,  with  a  ridiculously  large  head 
and  staring  eyes ;  still  there  is  no  difficulty  in  recognizing 
its  relationships.  During  the  next  week  internal  changes 
take  place.  The  shell  is  burst,  and  the  grasshopper  emerges, 
looking  more  like  its  parents  than  before.  This  process  is 
repeated  four  or  five  times  during  the  next  few  weeks,  and 
the  gradual  changes  thus  produced  finally  bring  the  young 
insect  to  the  adult  form.  This  latter  state  has  been  attained 
by  an  incomplete  metamorphosis. 


ARTHROPODS.    CLASS  INSECTS 


127 


In  the  flies,  beetles,  butterflies,  and  numerous  insects 
the  differences  between  the  newly  hatched  young  and  the 
adult  are  vastly  greater.  No  one  looking  on  a  caterpillar 
or  a  grub  for  the  first  time  would  suspect  its  origin,  and 
the  changes  they  undergo  have  attracted  attention  for  cen- 
turies. Placing  any  of  the  ordinary  caterpillars  with  their 
favorite  food  in  a  glass-covered  box,  we  may  readily  watch 
their  transformations.  Provided  with  biting  mouth-parts 
and  a  voracious  appetite,  they  devour  vast  quantities  of 
vegetation  for  several  days.  Finally  they  cease  eating,  and 


FIG.  79.— Life-history  of  silk-moth  (Bombyx  mori).    A,  adult ;  B,  C,  D,  caterpillars  of 
different  ages  ;  E,  F,  G,  silken  cocoon  and  pupa  ;  H,  eggs. 

suspend  themselves  head  downward  by  means  of  a  kind  of 
cobweb.  After  remaining  quiet  a  few  hours,  they  burst 
their  skin,  and  within  appears  a  chrysalis  or  pupa.  In  the 
moths,  for  example,  the  silk-moth  (Fig.  79),  the  caterpillar 
or  silk-worm,  after  eating  the  favorite  mulberry  leaves, 
spins  a  silken  cocoon,  in  which  the  pupa  is  produced.  The 
larvae  of  beetles  and  many  other  insects  excavate  tunnels  in 
wood  or  in  the  earth,  and  there  undergo  their  transforma- 
tions. Invariably  the  pupa  remains  quiet  for  days,  months, 
31 


128  ANIMAL  FORMS 

or  even  years,  but  when  the  proper  time  arrives  the  fully 
formed  insect  emerges,  and  takes  to  the  wing. 

Wonderful  internal  changes  have  been  taking  place 
during  this  time.  The  organs  fitted  for  the  proper  treat- 
ment of  the  vegetable  food  of  the  caterpillar  or  grub  are 
destroyed,  at  least  in  part,  and  new  systems  are  produced 
ready  for  the  nectar  and  vegetable  juices  which  are  to  be 
the  food  of  the  adult  insect.  All  insects  that  pass  through 
a  pupal  quiescent  stage  are  said  to  undergo  a  complete 
metamorphosis. 

124.  The  ants,  bees,  wasps,  etc.  (Hymenoptera). — The  ants, 
bees,  and  wasps  are  the  best-known  insects  belonging  to 
this  order.  They  are  characterized  by  four  membranous 
wings,  by  biting  and  sucking  mouth-parts,  and  the  female 
is  often  provided  with  a  sting.  All  undergo  a  complete 
metamorphosis.  The  eggs  may  be  laid  in  the  bodies  of 
other  insects,  many  of  which  are  pests,  and  are  thus  de- 
stroyed ;  or  they  may  be  deposited  in  the  nests  of  other 
insects,  the  foster-parents  being  compelled  to  feed  them ; 
or  they  may  be  placed  in  marvelously  constructed  homes, 
and  be  the  objects  of  the  greatest  attention,  the  parents  or 
attendants  often  risking  or  losing  their  lives  in  their 
defense.  The  members  of  this  order  have  long  attracted 
attention,  largely  on  account  of  their  remarkable  instinc- 
tive powers.  They  live  in  highly  organized  communities 
and  certain  of  their  characteristics  may  be  illustrated  by 
a  study  of  some  of  the  more  familiar  forms. 

126.  The  ants. — The  ants  live  in  communities  consisting 
of  anywhere  from  a  dozen  to  many  thousands  of  individuals, 
according  to  the  species.  Each  of  these  colonies  contains 
the  queen,  several  young  winged  males  and  females,  des- 
tined as  kings  and  queens  to  found  new  colonies,  and  of  a 
far  greater  number  of  wingless  sterile  females,  the  workers. 
The  workers  construct  the  greater  part  of  the  nest,  which 
often  consists  of  extensive  galleries,  nurseries,  and  grana- 
ries, excavated  in  wood  or  in  the  earth.  They  also  attend 


ARTHROPODS.    CLASS  INSECTS  129 

to  the  acquisition  of  food,  which  consists  of  the  sweet 
juices  of  plants,  of  other  insects,  or  of  leaves  and  seeds. 
These  may  be  fed  at  once,  or  placed  in  storehouses  until 
times  of  need. 

Certain  species  of  ants  make  carefully  planned  attacks 
upon  other  weaker  forms.  The  young  are  carried  off,  at 
times  only  after  a  prolonged  and  fierce  struggle,  and  all 
are  soon  eaten,  or  a  few  may  be  allowed  to  develop  and  act 
as  slaves.  Some  species  are  unable  to  exist  without  serv- 
ants, which  feed  them,  wash  them,  and  otherwise  minister 
to  their  comfort. 

In  some  of  their  raids  numerous  plant-lice  (delicate, 
usually  green,  insects,  such  as  occur  on  our  household 
plants)  are  often  captured  and  carried  into  the  nest.  These 
so-called  "  ant-cows "  are  carefully  tended,  and  in  return 
yield  up  a  tiny  drop  of  a  sugary  fluid  to  the  hungry  ant 
that  solicits  it. 

The  eggs  laid  by  the  queen  develop  into  white  worm- 
like  creatures,  which  ordinarily  spin  cocoons  when  about  to 
become  pupae.  These  are  incorrectly  called  "ant-eggs." 
Many,  probably  on  account  of  insufficient  nourishment, 
never  develop  reproductive  organs.  They  become  the  neu- 
ters or  workers.  The  winged  royalty  fly  away  from  the 
colony,  pair  and  found  homes  of  their  own,  and  become 
surrounded  by  a  numerous  progeny. 

126.  The  bees. — Among  the  bees  we  find  a  considerable 
number  which  lead  solitary  lives,  excavating  tunnels  in 
earth  or  wood,  as  in  the  case  of  many  of  the  wasps,  but, 
unlike  them,  supplying  the  young  with  honey  or  pollen. 
Others  may  constitute  a  band  of  worthless  insects  which 
steal  their  food  from  their  more  industrious  relations,  in 
whose  nests  they  also  secretly  deposit  their  eggs,  leaving  the 
young  to  be  nourished  with  food  rightly  belonging  to  others. 

But  it  is  with  the  social  bees  we  are  most  familiar — the 
bumble-  and  honey-bees.  The  former  usually  build  in  the 
ground,  and  form  colonies  consisting  of  the  queen  and  from 


130  ANIMAL  FORMS 

twenty  to  two  hundred  workers.  Kegular  combs  are  not 
constructed,  the  young  at  first  feeding  on  pollen  masses  or 
"bee-bread,"  and  finally  spinning  cocoons.  In  the  late 
summer  males  and  females  appear,  but  as  winter  comes  on 
all  perish  except  the  queens,  which  seek  a  sheltered  place, 
and  in  the  spring  revive  to  establish  new  colonies. 

In  a  wild  state  the  honey-bees  dwell  in  cavities  of  trees 
and  other  protected    places,  where  they  form    colonies, 

consisting  of  the  queen,  of  per- 
haps two  hundred  males  or 
drones  if  the  nest  be  examined 
in  the  spring  and  summer,  and 
of  a  hundred  times  as  many 
sterile  females,  the  workers. 
These  form  among  the  most 
highly  organized  insect  soci- 

80.-Bumblebee  (Bombus).  6tleS  kn°WIL      A11  WOI>k  f  Or  the 

good  of  the  colony.     To  each 

worker  is  assigned  a  definite  task,  which  is  never  shirked. 
It  must  collect  the  honey,  supply  the  wax  for  making  the 
comb,  take  care  of  the  brood,  or  in  other  ways  minister  to 
the  welfare  of  the  community.  On  the  queen  devolves 
the  entire  task  of  egg-laying.  She  may  lay  three  thou- 
sand eggs  a  day,  during  the  breeding  season,  for  the 
three  or  four  years  she  lives.  The  drones  or  males, 
after  one  nuptial  flight,  are  killed  or  driven  from  the 
hive  after  a  life  of  a  month  or  two.  The  unfertilized 
eggs  are  placed  in  large  cells,  and  the  young  fed  on 
pollen  develop  into  males.  The  fertilized  eggs  may  pro- 
duce queens  or  workers  at  the  discretion  of  the  queen.  If 
the  latter  be  desired,  the  eggs  are  placed  in  small  cells  with 
a  scant  amount  of  food,  which  apparently  causes  the  repro- 
ductive system  to  remain  undeveloped.  The  same  eggs,  if 
placed  in  the  large  queen  cells  and  supplied  with  highly 
nutritious  food,  would  have  developed  into  queens.  When 
these  latter  appear  they  are  vigorously  attacked  and  killed 


ARTHROPODS.    CLASS  INSECTS 


131 


by  the  parent  if  not  protected  by  the  workers.  If  the 
young  queen  survive,  the  old  queen  departs  with  many  of 
her  subjects,  and  collects  them  into  a  dense  swarm  attached 
to  a  limb  of  a  tree,  where  they  remain  until  scouts  return  to 
conduct  them  to  their  new  home. 

127.  The  wasps.— The  digger-wasps  are  frequently  to  be 
seen  gnawing  tunnels  in  the  wood  or  earth,  at  the  inner  end 


FIG.  81.— Nest  of  Vespa,  a  social  wasp.    Photograph  by  A.  L.  MBLANDEB  and 
C.  T.  BRUES. 

of  which  an  egg  is  laid.  In  some  species  the  developing 
young  is  nourished  by  food  carried  in  to  it  day  by  day.  In 
other  cases  the  parent  may  never  see  her  child,  dying  or 
abandoning  it  before  its  birth ;  but  before  departing  she  is 
careful  to  place  within  reach  a  sufficient  supply  of  spiders, 
caterpillars,  beetles,  or  locusts  that  shall  nourish  the  little 
one  until  it  becomes  a  motionless  pupa.  This  stage  is  soon 
over,  and  the  adult  wasp  now  digs  its  way  to  the  surface. 

Passing  by  the  familiar   mud-wasps   or  mud-daubers, 
whose  nests  are  common  objects  under  stones  or  against 


132  ANIMAL  FORMS 

the  rafters  of  barns  and  houses,  we  arrive  at  the  social 
wasps.  As  the  name  indicates,  these  insects,  such  as  the 
yellow-jackets  and  hornets,  live  together  in  companies 
which  consist,  as  in  the  ants  and  bees,  of  males,  females, 
and  workers.  They  also  are  fond  of  the  juices  of  fruits, 
and  many  of  them  destroy  insects  which  may  be  fed  to  the 
young.  Their  nests  are  variously  situated  and  constructed, 
but  all  of  them  agree  in  being  composed,  at  least  in  part,  of 
a  grayish  substance  which  is  in  reality  a  kind  of  paper. 
With  their  jaws  they  scrape  off  from  old  logs  and  fences 
small  particles  of  wood,  which  they  probably  mix  with  saliva, 
and  rolling  the  mass  into  a  ball  set  out  for  home.  These 
pellets  are  then  flattened  out  into  thin  sheets,  and  worked 
up  into  hexagonal  cells,  in  which  the  eggs  are  laid. 

Along  with  the  nests  of  the  mud-daubers  one  frequently 
notices  the  nests  of  some  of  the  familiar  wasps  (Polistes), 
which  build  cake-like  nests  composed  of  thirty  or  forty 
hexagonal  cells  attached  by  a  stalk.  Somewhat  similar 
nests,  though  usually  more  extensive,  are  constructed  by 
the  yellow- jackets  in  cavities  in  the  ground.  The  numer- 
ous combs  of  the  hornet  are  surrounded  by  several  sheets  of 
wood-pulp,  and  the  whole  structure  is  attached  generally 
to  the  limb  of  a  tree. 

In  the  spring  the  nests  of  all  these  species  of  wasps  are 
commenced  by  a  single  female,  who  has  lived  in  a  dormant 
condition  through  the  winter.  She  builds  a  small  nest  and 
in  time  is  surrounded  by  numerous  workers,  which  live  in 
perfect  harmony,  enlarging  the  nest  and  rearing  the  young. 
As  autumn  approaches  the  young  males  and  females  leave 
the  nest ;  but  the  males,  together  with  the  workers,  all  suc- 
cumb to  the  cold,  and  none  but  the  females  persist  to  found 
a  new  colony  the  following  spring. 


CHAPTER  XI 

ARTHROPODS  (Continued).     CLASS   ARACHNIDA 

128.  General  characters. — In  this  group,  comprising  the 
spiders,  mites,  and  a  large  assemblage  of  related  species,  we 
again  meet  with  great  differences  in  form  and  structure 
which  fit  them  for  lives  under  widely  different  conditions. 
The  three  regions  of  the  body,  head,  thorax,  and  abdomen, 
so  clearly  marked  in  the  insects,  are  here  less  plainly  de- 
fined. The  head  and  thorax  are  usually  closely  united,  and 
in  the  mites  the  boundaries  of  the  abdomen  are  also  indis- 
tinct. The  appendages  of  the  head  are  two  in  number,  and 
probably  correspond  to  the  antennae  and  mandibles  of  other 
Arthropods.  In  the  scorpions  and  some  species  of  mites 
these  are  furnished  with  pincers  for  holding  the  prey,  and 
in  other  forms  they  act  as  piercing  organs.  Usually  the 
thorax  bears  four  pairs  of  legs,  a  characteristic  which  readily 
separates  such  animals  from  the  insects. 

The  internal  organization  differs  almost  as  much  as  does 
the  external.  In  many  species  it  shows  a  considerable  re- 
semblance to  that  of  some  insects,  but  in  others,  especially 
those  of  parasitic  habits,  it  departs  widely  from  such  a  type. 
Respiration  is  affected  by  means  of  tracheae,  or  lung-books, 
which  consist  of  sacs  containing  many  blood-filled,  leaf -like 
plates  placed  together  like  the  leaves  of  a  book. 

Usually,  as  in  the  insects,  the  young  hatch  from  eggs 
which  are  laid,  but  in  the  scorpions  and  some  of  the  mites 
the  young  develop  within  the  body  and  at  birth  resemble 
the  parent.  Almost  all  of  these  organisms  live  either  as 

133 


134 


ANIMAL  FORMS 


parasites  or  as  active  predaceous  animals  upon  other  animals. 
For  this  purpose  many  are  provided  with  keen  senses  for 
detecting  their  prey  and  poisonous  spines  for  despatching  it. 
129.  The  scorpions. — Owing  to  the  stout  investing  armor, 
the  strong  pincers,  and  the  general  form  of  the  body,  the 
scorpions  might  at  first  sight  be  mistaken  for  near  relatives 

of  the  crayfish  or  lobster. 
A  more  careful  examina- 
tion will  show  that  the 
two  pairs  of  pincers  prob- 
ably correspond  to  the 
antennae  and  mandibles  of 
the  Crustacea  that  have 
become  modified  for  seiz- 
ing the  food.  The  swol- 
len part  of  the  animal 
lying  behind  the  four 
pairs  of  legs  is  a  part  of 
the  abdomen,  of  which 
the  slender  "  tail "  consti- 
tutes the  remainder.  On 
the  tip  of  the  tail  is  a 
curved  spine  supplied 
with  poison  glands.  Sev^ 
eral  pairs  of  eyes  are  borne 

FIG.  82.— Scorpion,  showing  pincer-like  mouth-  ,1         -,          -,  £  £ 

parts  ank  spine-tipped  tail.  °n    the    d°rsal   SUrface    °f 

the  head  and  thorax,  while 

on  the  under  side  of  the  animal  several  slit-like  openings 
lead  into  as  many  small  cavities  containing  the  lung-books. 
The  scorpions  are  the  inhabitants  of  warm  countries, 
where  they  may  be  found  under  sticks  and  stones  through- 
out the  day.  At  night  they  leave  their  homes  in  search  of 
food,  which  consists  chiefly  of  insects  and  spiders.  These 
are  seized  by  means  of  the  pincers,  and  the  sting  is  driven 
into  them  with  speedily  fatal  results.  It  is  doubtful  if  the 
poison  causes  death  in  man,  but  the  sting  of  some  of  the 


ARTHROPODS.    CLASS  ARACHN1DA  135 

larger  species,  which  measure  five  or  six  inches  in  length, 
may  produce  certain  disorders  chiefly  affecting  the  circula- 
tion. In  this  country  there  are  upward  of  thirty  species, 
most  of  which  are  comparatively  small. 

130.  The  harvestmen. — The  harvestmen  or  daddy-long- 
legs are  small-bodied,  long-legged  creatures  which  resemble 
in  general  appearance  several  of  the  spiders.     They  differ 
from  them,  however,  in  the  possession  of  claws  correspond- 
ing to  the  smaller  ones  of  the  scorpion,  and  in  their  method 
of  respiration,  which  is  similar  to  that  of  insects.     During 
the  day  they  conceal  themselves  in  dark  crevices  or  stride 
slowly  about  in  shaded  places ;  but  at  night  they  emerge 
into  more  open  districts  and  capture  small  insects,  from 
which  they  suck  the  juices. 

131.  The  spiders. — The  spiders  are  world-wide  in  their 
distribution,  and   are  a   highly  interesting   group,  owing 
chiefly  to  their  peculiar   habits.     Examining  any  of  our 
familiar  species,  it  will  be  seen  that  the  united  head  and 
thorax  are  separated  by  a  narrow  stalk  from  the  usually 
distended  abdomen.     To  the  under  side  of  the  former  are 
attached  four  pairs  of  long  legs,  a  pair  of  feelers,  and  the 
powerful  jaws  supplied  with  poison-sacs,  while  eight  shin- 
ing eyes  are  borne  on  the  top  of  the  head.     On  the  abdo- 
men, behind  the  last  pair  of  legs,  are  small  openings  into 
the  lung  cavities  which  contain  a  number  of  vascular,  leaf- 
like  projections  known  as  lung-books.     In  some  species 
a  well-marked  system  of  tracheae  are  also  present.     At  the 
hinder  end  of  the  body  are  four  or  six  little  projections, 
the   spinnerets,  each  of  which   is   perforated  with   many 
holes.     Through  these  the  secretion  from  the  glands  be- 
neath is  squeezed  out  in  the  form  of  excessively  delicate 
threads,  often  several  hundred  in  number,  which  harden  on 
exposure  to  the  air.     According  to  the  use  for  which  these 
are  intended,  they  may  remain  a  tangled  mass  or  become 
united  into  one  firm  thread ;  and  according  to  the  habits 
of  the  animal,  they  may  be  used  for  enclosing  their  eggs, 


136  ANIMAL  FORMS 

for  lining  their  burrows,  or  for  the  construction  of  webs  of 
the  most  diverse  patterns. 

132.  The  habits  of  spiders. — Many  species  of  spiders,  some 
of  which  are  familiar  objects  in  fields  and  houses,  construct 
sheets  of  cobweb  with  a  tube  at  one  side  in  which  they  may 


FIG.  83.— A  tarantula-spider  (Eurypelma  lentzii).    Natural  size.    Photograph  by 
A.  L.  MELANDER.and  C.  T.  BRUES. 

lie  in  wait  for  their  prey  or  through  which  they  may  escape 
in  times  of  danger.  In  the  webs  of  the  common  orb-  or 
wheel-weavers  several  radial  lines  are  first  constructed,  and 
upon  these  the  female  spider  spins  a  spiral  web.  Kesting 
in  the  center  of  this  or  at  the  margin,  with  her  foot  on 
some  of  the  radial  threads,  she  is  able  to  detect  the  slight- 
est tremor  and  at  once  to  rush  upon  the  entangled  captive. 
Some  of  the  bird-spiders  and  their  allies,  living  in  trop- 
ical America,  and  attaining  a  length  of  two  inches,  con- 
struct web-lined  burrows  in  the  ground.  From  these  they 
stalk  their  prey,  which  consists  of  various  insects  and  even 


ARTHROPODS.    CLASS  ARACHNIDA 


137 


small  birds.  These  are  almost  instantly  killed  by  the  poison- 
fangs,  and  are  then  carried  to  the  burrow,  where  the  juices 
of  the  body  are  extracted. 

The  trap-door  spiders  of  the  southwestern  section  of  the 
United  States  also  dig  tunnels,  which  they  cover  with  a 
closely  fitting  lid  com- 
posed of  earth.  Eaising 
this  they  come  out  IB 
search  of  insects,  but  if 
sought  in  turn,  they  dash 
into  the  burrow,  closing 
the  door  after  them,  and 
holding  it  with  such  firm- 
ness that  it  is  rarely  forced 
open.  If  this  should  hap- 
pen, there  are  sometimes 
blind  passage-ways,  also 
closed  with  trap-doors, 
which  usually  baffle  the 
pursuer. 

Finally,  there  are 
among  the  thousand  spe- 
cies of  spiders  in  the  United  States  a  considerable  propor- 
tion which  construct  no  definite  web.  Many  of  these  may 
be  seen  darting  about  in  the  sunshine  on  old  logs  and 
fences,  often  trailing  after  them  a  thread  which  may  sup- 
port them  if  they  fall  in  their  active  leaping  after  in- 
sects. 

133.  Breeding  habits. — The  male  spiders  are  usually  much 
smaller  than  the  females,  and  some  species  are  only  one- 
fifteenth  as  long  as  the  female  and  one  one-hundredth  of 
its  weight.  They  are  usually  more  brilliantly  colored,  more 
active  in  their  movements,  yet  rarely  spinning  their  own 
webs  and  capturing  their  own  food,  preferring  to  live  at 
the  expense  of  the  female.  At  the  breeding  season  the 
males  of  several  species^  make  a  most  interesting  display 


Fio.  84. — Trap-door  spider  and  burrow 
( Ctenizd). 


138  ANIMAL  FORMS 

of  their  colors,  activity,  and  gracefulness  before  the  females ; 
and  the  latter,  after  watching  these  exhibitions,  are  said  to 
select  the  one  who  has  "  shown  off  "  in  the  most  pleasing 
fashion.  The  life  after  this  may  be  stormy,  resulting  in 
the  death  of  the  male ;  but  ordinarily  the  results  are  not 
so  disastrous,  and  in  a  little  while  the  female  deposits  her 
eggs  in  cases  which  she  spins.  In  these  the  young  develop, 
sometimes  wintering  here,  and  emerging  in  the  spring  to 
scamper  about  in  search  of  food,  or  to  drift  through  the 
air  to  more  favorable  spots  on  fluffy  masses  of  cobweb. 

Few  groups  of  animals  are  more  interesting  objects  of 
study  and  more  accessible.  Their  bites  are  rarely  more 
serious  than  those  of  the  mosquito — never  fatal ;  and  a 
careful  study  of  any  species,  however 
common,  will  undoubtedly  bring  to 
light  many  interesting  and  unknown 
facts. 

134.  The  mites  and  ticks.— The 
mites  and  ticks  are  the  simplest  and 
among  the  smallest  of  the  animals 
belonging  to  this  group.  To  the  at- 
tentive observer  they  are  rather  com- 
mon objects,  with  homes  in  very  dif- 
ferent situations.  Some  occur  on  liv- 
FIG.  85.-The  itch-mite  (Sar-  ing  and  decaying  vegetation,  in  old 

copies  scabet).  s  n      i  i_"i 

flour  and  unrefined  sugar,  while  oth- 
ers live  in  fresh  water  and  a  few  in  the  sea.  Almost  all 
tend  toward  _  parasitism.  Some  of  the  insects  which  they 
pierce  and  destroy  are  a  pest  to  man,  but  on  the  other  hand 
some  are  intolerable  owing  to  the  diseases  they  produce. 

As  to  other  parasitic  organisms,  degradation  of  structure 
is  manifest.  The  respiratory  system,  so  important  to  the 
active  life  of  the  insects,  may  be  absent,  the  animal  breath- 
ing through  its  skin.  The  circulatory  system  may  be  want- 
ing, the  blood  occupying  spaces  among  the  various  organs 
being  swept  about  by  the  animal's  movements.  And  many 


ARTHROPODS.  CLASS  ARACHNIDA 


139 


other  peculiarities   have  arisen  which  fit  them  for  their 
different  modes  of  life. 

135.  The  king  crab  (Limulus). — The  king  crab  may  be 
found  crawling  over  the  bottom  or  plowing  its  way  through 
the  sand  and  mud  in  many  of  the  quiet  bays  from  Maine 
to  Florida.  The  large  head  and  thorax  of  these  animals 
are  united  into  a  horse- 
shoe-shaped piece,  be- 
hind which  lies  the 
triangular  abdomen. 
On  the  curved  front 
surface  of  the  former 
are  a  pair  of  small  me- 
dian eyes,  and  farther 
outward  are  two  larger 
compound  ones.  On 
the  ventral  side  are 
six  pairs  of  append- 
ages, instrumental  in 
capturing  and  tearing 
the  small  animals  that 
serve  as  food,  and 
functioning  in  con- 
nection with  the  ter- 
minal spine  as  locomo- 
tor  organs.  On  the 
ventral  surface  .of  the  abdomen  are  numerous  plate-like  flaps 
which  serve  in  respiration,  and  in  the  imperfect  swimming 
movements  in  which  these  animals  occasionally  indulge. 

These  relatively  large  and  clumsy  creatures  are  the  rem- 
nant of  a  great  number  of  strange,  uncouth  animals  that  in- 
habited the  earth  in  past  ages.  Many  of  them  show  a  close 
resemblance  to  the  scorpions.  The  anatomy  and  develop- 
ment also  show  certain  points  of  resemblance,  and  by  some 
are  thought  to  give  us  an  idea  of  the  ancient  type  of  spider- 
like  animal  from  which  the  modern  forms  have  descended. 


FIG.  86.— The  king  or  horseshoe  crab  (Limulus 
polyphemus). 


CHAPTER  XII 

ECHINODERMS 

136.  General  characters.— The  division  of  the  echino- 
derms  includes  the  starfishes,  sea-urchins,  serpent-  or  brittle- 
stars,  sea-cucumbers,  and  crinoids  or  sea-lilies.    All  are  ma- 
rine forms,  and  constitute  a  conspicuous  portion  of  the 
animals  along  almost  any  coast  the  world  over.     From 
these  shallow-water  situations  they  extend  to  the  greatest 
depths  of  the  ocean,  and  the  bodily  form  possesses  a  great 
number  of  variations,  adapting  them  to  lives  under  such 
diverse  conditions;  and  yet  there  is  perhaps  no  group  of 
organisms  so  clearly  denned  or  exhibiting  so  close  a  resem- 
blance throughout.     At  one  time  it  was  thought  that  their 
radial  symmetry  was  an  indication  of  a  close  relationship 
to  the  coelenterates,  but  more  careful  study  has  shown  them 
to  be  much  more  highly  developed  than  this  latter  group, 
and  widely  separated  from  it.     A  skeleton  is  almost  always 
present,  consisting  of  a  number  of  calcareous  plates  embed- 
ded in  the  body-wall,  and  often  supporting  numbers  of  pro- 
tective spines,  which  fact  has  given  to  the  group  the  name 
Echinoderm,  meaning  hedgehog  skin. 

137.  External  features.— The  body  of  a  starfish  (Fig.  87) 
consists  of  a  more  or  less  clearly  defined  disk,  from  which 
the  arms,  usually  five  in  number,  radiate  like  the  spokes 
of  a  wheel.     At  the  center  of  the  under  side  the  mouth  is 
located,  and  from  it  a  deep  groove,  filled  with  a  mass  of 
tubular  feet,  extends  to  the  tip  of  each  arm.     Innumerable 
calcareous  plates  firmly  embedded  in  the  body-wall  serve 

140 


ECHINODERMS 


141 


for  the  protection  of  the  internal  organs,  and  at  the  same 
time  admit  of  considerable  movement. 

In  the  brittle-stars  (Fig.  88)  the  central  disk  is  much 
more  sharply  defined  than  in  the  preceding  forms,  and  the 
long  snake-like  arms  are  capable  of  a  very  great  freedom  of 
movement,  enabling  the  animal  to  glide  over  the  sea-bottom, 
or  through  the  crevices  of  the  rocks,  at  a  surprising  rate. 

In  several  species,  otherwise  closely  resembling  those 


FIG.  87.— Starfish  (Asterias  ocracea),  ventral  view.    One-half  natural  size. 

in  Fig.  88,  the  arms  divide  repeatedly.  These  are  the  so- 
called  basket-stars,  living  in  the  deeper  waters  of  the  sea, 
where  they,  like  other  brittle-stars,  act  as  scavengers  and 
devour  large  quantities  of  decomposing  plant  or  animal 
remains. 

At  first  sight  the  globular  spiny  sea-urchins  (Fig.  90) 
would  scarcely  be  recognized  as  close  relatives  of  the  star- 
fishes. A  closer  examination,  however,  shows  the  mouth  to 
be  located  on  the  under  side  of  the  body ;  from  it  five  rows 
of  feet  radiate  and  terminate  close  to  the  center  of  the 
dorsal  side,  and  the  arrangement  of  the  plates  forming  the 


142 


ANIMAL  FORMS 


skeleton  indicate  that  the  sea-urchin  is  comparable  to  a 
starfish,  with  its  dorsal  surface  reduced  to  insignificant 
proportions. 

In  the  sea-urchins  the  calcareous  plates  possess  a  great 
regularity,  and  are  so  closely  interlocked  that  they  prevent 


FIG.  88.— Brittle-  or  serpent-stars  (species  undetermined).    Natural  size. 

any  motion  of  the  body-wall.  Also,  each  plate  is  usually 
provided  with  highly  developed  spines,  movable  upon  a  ball- 
and-socket  joint.  These  spines  serve  for  locomotion,  and, 
in  some  instances,  for  conveying  food  to  the  mouth.  A 
considerable  number  of  sea-urchins  show  an  irregularity  in 
form  which  destroys  to  a  corresponding  degree  the  radial 
symmetry.  This  is  due  to  various  causes,  but  especially  to 
a  compression  of  the  body,  which,  in  the  "sand-dollars," 


ECHINODERMS  143 

has  resulted  in  the  production  of  a  thin,  cake-like  form 
(Fig.  91). 

If  the  spherical  body  of  a  sea-urchin  were  to  be  stretched 
in  the  direction  of  a  line  joining  the  mouth  and  the  center 


FIG.  89.— Basket-star  (Astrophyton).    One-half  natural  size. 

of  the  dorsal  surface,  a  form  resembling  a  sea-cucumber 
(Fig.  92)  would  be  the  result.  These  latter  organisms  live 
among  crevices  of  the  rocks,  embedded  in  the  mud  or  bur- 
rowing in  the  sand  at  the  bottom  of  the  sea.  In  such  situa- 
tions they  are  well  protected,  and  a  highly  developed  skele- 
ton, such  as  that  of  the  sea-urchin,  would  not  only  be  of 
little  value,  but  a  positive  hindrance  to  locomotion.  The 
skeleton,  therefore,  is  much  reduced,  consisting  of  a  few 
scattered  calcareous  plates  embedded  in  the  fleshy  body- 
wall.  Another  peculiar  feature  is  almost  universally  pres- 
ent, in  the  form  of  a  circlet  of  tentacles  surrounding  the 
mouth,  which  serve  either  for  the  purpose  of  respiration, 
for  locomotion,  or  to  convey  food  to  the  mouth. 

A  very  good  imitation  of  the  general  plan  of  a  sea-lily 
or  crinoid  (Fig.  93)  could  be  made  by  attaching  a  serpent- 


144 


ANIMAL  FORMS 


star,  especially  one  of  the  basket-stars,  by  its  dorsal  side 
to  a  stalk.     In  the  crinoids  the  numerous  branches  of  the 

arms  are  compara- 
tively short,  and  in 
the  arrangement  of 
the  internal  organs 
there  are  numer- 
ous differences,  but 
for  all  that  the  re- 
semblance of  these 
organisms  to  the 
other  echinoderms 
is  undoubted. 

138.  Haunts, — 
The  greater  num- 
ber of  starfishes 
occur  alongshore, 
slowly  crawling 
about  in  search  of 
food,  or  concealed 
in  dark  crevices  of 
the  rocks,  where  they  may  often  be  found  as  the  tide  goes 
out,  and  we  know  that  in  gradually  lessening  numbers  other 
species  lead  similar  lives  at  different  levels  far  down  in  the 
dark  and  gloomy  depths.  In  these  same  locations  the  sea- 
urchins  occur,  sometimes  singly,  but  more  usually  associa- 
ted in  great  numbers,  several  species  excavating  hollows  in 
the  rocks,  within  which  they  obtain  protection.  The  brit- 
tle-stars and  sea-cucumbers  may  also  be  found  occasionally 
in  open  view,  but  more  often  they  make  their  way  about  in 
search  of  food  buried  in  the  sand.  The  crinoids  are  usual- 
ly inhabitants  of  deeper  water,  where  they  are  found  asso- 
ciated often  in  great  numbers.  A  few  species  upon  attain- 
ing the  adult  condition  separate  from  the  stalk,  and  are 
able  to  move  about  (Fig.  94),  but  the  remaining  species 
never  shift  their  position. 


FIG.  90.— Sea-urchin  (Strongylocentrotus  purpuratus). 
Natural  size. 


ECHINODERMS  145 

139.  The  organs  of  defense  and  repair  of  injury.— As  we 

have  seen,  the  body-wall  of  the  echinoderms  is  provided 
with  a  series  of  plates,  often  bearing  spines  which  serve  as 
organs  of  defense,  and  to  protect  the  internal  organs.  The 
starfishes  and  sea-urchins  also  possess  numerous  modified 
spines  (pedicellaria)  scattered  over  the  surface  of  the  body, 
which  have  the  form  of  miniature  birds'  beaks,  fastened  to 
slender  muscular  threads.  During  life  these  jaws  continu- 
ally open  and  close,  and  it  is  said  they  clean  the  body  of 
debris  that  settles  on  it ;  but  on  the  other  hand  there  are 
several  reasons  for  the  belief  that  they  also  act  as  organs 
of  defense.  Thus  protected,  the  natural  enemies  of  echino- 
derms appear  to  be  relatively  few,  and  are  confined  chiefly 
to  some  of  the  fishes  whose  teeth  are  especially  modified 
for  crushing  them.  In  this 
way,  and  owing  to  the  action 
of  the  breakers,  they  suffer 
frequent  injury,  but  many 
species  exhibit  to  a  remark- 
able degree  the  ability  to  re- 
generate lost  parts.  Experi- 
ments show  that  if  all  the 
arms  of  a  starfish  be  separa- 
ted from  the  disk  the  latter 
will  within  two  or  three  *^^^^^ 

months  renew  the  arms  ;  and    FlG-  w-sand-doiiar,  B  flat  sea-urchin. 

Natural  size. 

a  single  arm  with  a  part  of 

the  disk  is  able  to  renew  the  missing  portions  in  about  the 

same  length  of  time. 

The  brittle-stars,  as  their  name  indicates,  are  usually  ex- 
cessively delicate,  often  dropping  all  of  their  arms  upon  the 
slightest  provocation ;  but  here  again  the  ability  is  present 
to  develop  the  lost  portions. 

Sea-cucumbers  resent  rough  treatment  by  vigorously 
contracting  their  muscular  walls  and  removing  from  the 
body  almost  the  entire  digestive  tract,  the  respiratory  tree, 


146 


ANIMAL  FORMS 


and  a  portion  of  the  locomotor  system ;  but  some  species,  at 
least,  renew  them  again.  In  some  of  the  starfishes  and 

brittle-stars  portions  of  the  body 
appear  to  be  voluntarily  de- 
tached and  to  develop  into  new 
individuals,  and  it  is  thought 
that  such  self-mutilation  is  a 
normal  method  of  reproduction. 
140.  Locomotor  system. — One 
of  the  most  characteristic  and 
remarkable  features  of  the  echi- 
noderms  is  the  water-vascular 
system,  a  series  of  vessels  con- 
taining water  which  serve  in  the 
process  of  locomotion.  Their 
arrangement  and  mode  of  opera- 
tion are,  with  slight  modifica- 
tions, the  same  throughout  the 
group,  and  may  be  readily  un- 
derstood from  their  study  in 
the  starfish. 

On  the  dorsal   surface  of  a 
starfish,  in  the   angle  between 

two  of  the  arms,  is  a  round,  slightly  elevated,  calcareous 
plate,  the  madreporic  body  (Fig.  95,  m.p.),  which  under 
the  microscope  appears  full  of  holes,  like  the  "  rose  "  of  a 
watering-pot.  This  connects  with  a  tube  that  passes  to 
the  opposite  side  of  the  body,  where  it  enters  a  canal 
completely  encircling  the  mouth.  On  this  ring-canal  a 
number  of  sac-like  reservoirs  with  muscular  walls  are  at- 
tached, and  from  it  a  vessel  extends  along  the  under  sur- 
face of  each  arm  from  base  to  tip.  Each  of  these  radial 
water-mains  gives  off  numerous  lateral  branches  that  open 
out  into  small  reservoirs  similar  to  those  located  on  the 
ring-canal,  and  a  short  distance  beyond  communicate 
through  the  wall  of  the  body  with  one  of  the  numerous 


FIG.  92.— Sea-cucumber  (Cucu- 
maria  sp.).    Natural  size. 


ECHINODERMS 


147 


tube-feet,  which,  as  we  have  seen,  are  slender  tubular  or- 
gans, many  in  number,  filling  the  grooves  on  the  ventral 
surface  of  each  arm.  This  entire  system  of  tubes  and 
reservoirs  is  full  of  .water,  taken  in,  it  is  said,  through  the 
perforated  plate,  and,  when  the  starfish  wishes  to  advance, 
many  of  the  little  reservoirs  con- 
tract, forcing  water  into  the  cav- 
ity of  the  feet,  with  which  they 
are  in  communication,  thus  ex- 
tending the  extremity  of  the  tubes 
a  considerable  distance.  The 
terminal  sucker  of  each  foot,  act- 
ing upon  the  same  principle  as 
those  on  the  cuttlefish,  attaches 
firmly  to  some  foreign  object, 
whereupon  the  muscles  of  the 
foot  contract,  drawing  the  body 
toward  the  point  of  attachment. 
This  latter  movement  is  similar 
to  that  of  a  boatman  pulling  him- 
self to  land  by  means  of  a  rope 
fastened  to  the  shore.  When  the 
shortening  of  the  tube-feet  has 
ceased,  the  sucking  disks  release 
their  attachment,  project  them- 
selves again,  and  this  process  is 
repeated  over  and  over.  At  all 
times  some  of  the  feet  are  con- 
tracting, and  a  steady  advance  of 
the  body  is  the  result. 

This   method    of    locomotion 

also  obtains  in  the  sea-urchins  and  cucumbers,  but  in  the 
serpent-stars  the  tube-feet  have  become  modified  into  feel- 
ers, and  the  animal  moves,  often  rapidly,  by  means  of  twist- 
ing movements  of  the  arms.  The  feet  have  this  character 
also  in  the  crinoids,  where  the  animal  is  generally  without 


FIG.  93. — Sea-lily  or  crmoicl. 


148  ANIMAL  FORMS 

the  power  of  locomotion.     In  some  of  the  sea-cucumbers 
five  equidistant  rows  of  tube-feet  extend  from  one  end  of 
the  body  to  the  other,  and  the  animal  crawls  worm-like 
upon  any  side  that  happens  to  be  down ;  but  certain  spe- 
cies living  in  the  sand, 
where    tube  -  feet    will 
not  work  satisfactorily, 
have  lost  all  traces  of 
them,  and  creep  like  an 
earthworm  from  place 
to  place.      In  all    the 
sea-cucumbers  the  feet, 
situated  near  the  mouth, 
have     been     curiously 
modified  to  form  a  cir- 

FIG.  94.— An  unattached  crinoid  (Antedon).    One-  . 

half  natural  size.  clet  OI  tentacles,  which 

range  in  form  from 

highly  branched  to  short  and  thick  structures,  and  in  func- 
tion from  respiratory  organs  and  those  of  touch  to  con- 
trivances for  scooping  up  sand  and  conveying  it  to  the 
mouth. 

141.  Food  and  digestive  system. — In  the  echinoderms  the 
body-wall  is  comparatively  thin  (Fig.  95),  and  encloses  a 
great  space,  the  body-cavity,  in  which  the  digestive  and  re- 
productive organs  are  contained.  As  the  former  in  various 
species  is  adapted  for  acting  upon  very  different  kinds  of 
food,  it  shows  many  modifications ;  but  there  are  a  few  prin- 
cipal types  which  may  be  briefly  considered. 

In  the  starfishes  the  mouth  enters  almost  directly  into 
the  cardiac  division  of  the  stomach,  a  capacious,  thin-walled 
sac,  much  folded  and  packed  away  in  the  disk  and  bases  of 
the  arms  (Fig.  95,  £).  This  in  turn  leads  into  the  second 
pyloric  portion  (#),  with  thicker  walls  and  dorsal,  to  the 
first,  from  which  a  short  intestine  leads  to  the  exterior, 
near  the  center  of  the  disk.  Another  conspicuous  and  im- 
portant feature  is  the  so-called  liver,  consisting  of  a  pair 


ECHINODERMS 


149 


of  closely  branched,  fluffy  glands  (Z),  extending  the  entire 
length  of  each  arm  and  opening  into  the  pyloric  stomach. 

The  starfishes  are  carnivorous  and  highly  voracious,  de- 
vouring large  numbers  of  barnacles  and  mollusks  which  hap- 
pen in  their  path.  If  these  are  small  and  free  they  are 
taken  directly  into  the  stomach,  but  when  one  of  relatively 
large  size  is  encountered  the  starfish  settles  down  upon  it, 
and,  slowly  pushing  the  cardiac  stomach  through  the  mouth, 
envelops  it  in  the  folds.  Digestive  fluids  are  now  poured 
over  it,  and  the  victim  is  speedily  despatched  and  in  a  partly 
digested  condition  is  gradually  absorbed  into  the  body,  leav- 


FIG.  95.— Dissection  of  starfish  to  show  :  a,  pyloric  stomach  ;  b,  bile-ducte  (above), 
cardiac  stomach  (be,low) ;  b.c.,  body-cavity;  /,  feet;  g,  spines;  i,  intestine; 
/,  liver;  m,  mouth;  m.p.,  madre'poric  plate  ;  *r,  reservoir;  r.c.,  ring  canal; 
r.m.,  stomach  retractor  muscle  ;  r.v.,  radial  vessel ;  «,  stone  canal ;  t,  respira- 
tory tree. 

ing  the  shell  and  other  indigestible  matters  upon  the  exte- 
rior. Oysters  and  clams  close  their  shells  when  thus  attacked, 
but  a  steady,  continuous  pull  on  the  part  of  the  starfish 
finally  opens  them,"  and  the  stomach  is  spread  over  the  fleshy 
portions  with  speedily  fatal  results.  In  the  interior  of  the 
body  the  food  is  transferred  to  the  pyloric  stomach,  sub- 
jected to  the  action  of  the  liver,  and  when  completely  dis- 
solved is  borne  to  all  parts  of  the  body. 


150  ANIMAL  FORMS 

The  digestive  system  of  the  starfishes,  with  its  various 
subdivisions  and  appendages,  is  in  some  respects  more  com- 
plicated than  in  the  other  classes.  This  is  most  strikingly 
the  case  with  the  serpent-stars,  where  the  entire  system  for 
disposing  of  the  minute  animals  and  plants  on  which  it 
feeds  consists  of  a  simple  sac  communicating  with  the 
exterior  by  a  single  opening — the  mouth. 

In  the  sea-cucumbers  large  quantities  of  sand  are  taken 
into  the  body,  and  the  minute  organisms  and  organic  mat- 
ter are  digested  from  it.  In  the  sea-urchins  the  mouth  is 
provided  with  five  teeth,  and  the  food  consists  of  minute 
bits  of  seaweeds,  which  these  snip  off.  Such  diets  evidently 
require  a  comparatively  simple  digestive  apparatus,  for  in 
both  it  consists  throughout  its  whole  extent  of  a  tube  of 
equal  caliber,  in  which  the  various  divisions  of  esophagus, 
stomach,  and  intestine  are  little,  if  at  all,  defined.  This 
is  usually  somewhat  longer  than  the  body,  and  therefore 
thrown  into  several  loops ;  and  in  the  sea-cucumbers  its  last 
division  is  expanded  and  furnished  with  more  highly  mus- 
cular walls,  which  aid  in  respiration. 

142.  Development. — With  but  a  few  exceptions,  the  eggs 
of  the  echinoderms  are  laid  directly  in  the  surrounding 
water,  and  for  many  days  the  exceedingly  minute  young 
are  borne  great  distances  in  the  tidal  currents.  During 
this  period  they  show  no  resemblance  to  their  parents,  and 
only  after  undergoing  remarkable  transformations  do  they 
assume  their  permanent  features.  In  every  case  they  have 
a  five-rayed  form  in  early  youth,  but  in  several  species  of 
starfishes  additional  arms  develop  until  there  may  be  as 
many  as  twenty  or  thirty. 


CHAPTEE    XIII 

THE    CHORDATES 

143.  General  characters.— Up  to  the  present  time  we  have 
been  studying  the  representatives  of  a  vast  assemblage  of 
animals  whose  skeletons,  if  they  have  any  at  all,  are  located 
on  the  outside  of  the  body.  In  the  corals,  the  mighty  com- 
pany of  arthropods,  and  the  echinoderms,  it  is  external.  On 
the  other  hand,  we  shall  find  that  the  animals  we  are  now 
about  to  consider,  the  fishes,  frogs,  lizards,  birds,  and  mam- 
mals, are  in  possession  of  an  internal  skeleton.  In  some  of 
the  simpler  fishes  and  in  a  number  of  more  lowly  forms  (Fig. 
96)  it  is  exceedingly  simple,  and  consists  merely  of  a  gristle- 
like  rod,  the  notochord  (Fig.  98,  nc),  extending  the  length 
of  the  body  and  serving  to  support  the  nervous  system,  which 
is  always  dorsal.  This  is  also  the  type  of  skeleton  found  in 
the  young  of  the  remaining  higher  animals,  but  as  they  grow 
older  the  notochord  gives  way  to  a  more  highly  developed 
cartilaginous  or  bony,  jointed  skeleton,  the  vertebral  column. 

In  the  young  of  all  these  back-boned  or  chordate  ani- 
mals, the  sides  of  the  throat  are  invariably  perforated  to 
form  a  number  of  gill-slits.  In  the  lower  forms  these  per- 
sist and  serve  as  respiratory  organs,  but  in  the  higher  ani- 
mals they  disappear  in  the  adult.  The  chordates  are  thus 
seen  to  be  distinguished  by  the  possession  of  a  dorsal  nerv- 
ous cord  supported  by  an  internal  skeleton  and  by  the 
presence  of  gill-slits,  characters  which  separate  them  widely 
from  all  invertebrates. 

The  chordates  may  be  divided  into  ten  classes,  seven  of 

151 


152 


ANIMAL  FORMS 


which  (the  lancelets,  lampreys,  fishes,  amphibians,  reptiles, 
birds,  and  mammals)  are  true  vertebrates,  while  the  others 
embrace  several  peculiar  animals  of  much  simpler  organiza- 
tion. 

144.  The  ascidians. — Among  the  latter  are  a  number  of 
remarkable  species  belonging  to  the  class  of  ascidians  or 

sea-squirts  (Fig.  96). 
These  are  abundantly 
represented  along  our 
coasts,  and  are  readily 
distinguished  by  their 
sac -like  bodies,  which 
are  often  attached  at 
one  end  to  shells  or 
rocks.  On  the  opposite 
extremity  two  openings 
exist,  through  which  a 
constant  stream  of  water 
passes,  bearing  minute 
organisms  serving  as 
food.  When  disturbed 
they  frequently  expel 
the  water  from  these 
pores  with  considerable 
force,  whence  the  name 
"  sea-squirt."  While 
many  lead  solitary  lives, 

numerous  individuals  of  other  species  are  often  closely 
packed  together  in  a  jelly-like  pad  attached  to  the  rocks, 
and  others  not  distantly  related  are  fitted  to  float  on  the 
surface  of  the  sea. 

The  young  when  hatched  resemble  small  tadpoles  both  in 
their  shape  and  in  the  arrangement  of  some  of  the  more 
important  systems  of  organs.  For  a  few  hours  each  swims 
about,  then  selecting  a  suitable  spot  settles  down  and  ad- 
heres for  life.  From  this  point  on  degeneration  ensues, 


FIG.  ,96.— Ascidian  or  sea-squirt. 


THE  CHORDATES  153 

The  tail  disappears,  and  with  it  the  notochord  and  the 
greater  part  of  the  nervous  system.  The  sense-organs  van- 
ish, the  pharynx  becomes  remodeled,  and  numerous  other 
changes  occur,  leaving  the  animal  in  its  adult  condition, 
with  little  in  its  motionless,  sac-like  body  to  remind  one  of 
a  vertebrate. 

145.  The  vertebrates. — Since  the  remainder  of  this  vol- 
ume is  concerned  with  the  vertebrates  it  will  be  well  at  the 
outset  to  gain  some  knowledge  of  their  more  important 
characteristics.  One  of  the  most  apparent  is  the  presence 
of  a  jointed  vertebral  column,  composed  of  cartilage  or 
bone,  which  supports  the  nervous  system.  To  it  are  also 
usually  attached  several  pairs  of  ribs,  two  pairs  of  limbs, 
either  fins,  legs,  or  wings,  and  in  front  it  terminates  in  a 
more  or  less  highly  developed  skull.  In  the  space  par- 
tially  enclosed  by  the  ribs,  the  body-cavity,  a  digestive  sys- 
tem is  located,  which  consists  of  the  stomach  and  intestine^ 
together  with  the  attached  liver  and  pancreas.  The  cir- 
culatory system  is  also  highly  organized,  and  consists  of  a 
muscular  heart,  arteries,  and  veins  which  ramify  through- 
out the  body.  Breathing,  in  the  aquatic  animals,  is  car- 
ried on  by  means  of  gills,  and  in  the  air-breathing  forms 
by  means  of  lungs,  which,  like  the  gills,  effect  the  removal 
of  carbonic-acid  gas  and  the  absorption  of  oxygen.  The 
nervous  system,  consisting  of  the  brain  situated  in  the 
head  and  the  spinal  cord  extending  through  the  body 
above  the  back-bone,  even  in  the  lower  vertebrates,  is  far 
more  complex  than  in  the  invertebrates.  The  sense-organs 
also  attain  to  a  high  degree  of  acuteness,  and  in  connec- 
tion with  the  highly  organized  nervous  system  enable  these 
forms  to  lead  far  more  varied  and  complex  lives  than  in 
any  of  the  animals  heretofore  considered. 


CHAPTER  XIV 

THE   FISHES 

146.  General  characters, — In  a  general  way  the  name 
fish  is  applied  to  all  vertebrates  which  spend  the  whole 
of  their  life  in  the  water,  which  undergo  no  retrograde 
metamorphosis,  and  which  do  not  develop  fingers  or  toes. 
Of  other  aquatic  chordates  or  vertebrates  the  ascidians  un- 
dergo a  retrograde  metamorphosis,  losing  the  notochord,  and 
with  it  all  semblance  of  fish-like  form.  The  amphibians, 
on  the  other  hand,  develop  jointed  limbs  with  fingers  and 
toes,  instead  of  paired  fins  with  fin  rays.  A  further  com- 
parison of  the  animals  called  fishes  reveals  very  great  dif- 
ferences among  them — differences  of  such  extent  that  they 
cannot  be  placed  in  a  single  class.  At  least  three  great 
groups  or  classes  must  be  recognized :  the  Lancelots,  the 
Lampreys,  and  the  True  Fishes.  The  general  characters  of 
all  these  groups  will  be  better  understood  after  the  study 
of  some  typical  fish,  that  is  one  possessing  as  many  fish-like 
features  as  possible,  unmodified  by  peculiar  habits.  Such  an 
example  is  found  in  the  bass,  trout,  or  perch.  In  either  fish 
the  pointed  head  is  united,  without  any  external  sign  of  a 
neck,  to  the  smooth,  spindle-shaped  body,  which  is  thus  fitted 
for  easy  and  rapid  cleaving  of  the  water  when  propelled  by 
the  waving  of  the  powerful  tail  (Fig.  97).  A  keel  also  has 
been  provided,  enabling  the  fish  to  steer  true  to  its  course. 
This  consists  of  folds  of  skin  arising  along  the  middle  line  of 
the  body,  supported  by  numerous  bony  spines  or  cartilaginous 
154 


THE  FISHES  155 

rays.  These  are  the  unpaired  fins,  as  distinguished  from 
the  paired  ones,  which  correspond  to  the  limbs  of  the  higher 
vertebrates.  In  the  bass  or  perch  the  latter  are  of  much 
service  in  swimming,  and  are  also  most  important  organs  in 
directing  the  course  of  the  fish  upward  or  downward,  or  for 


Pf 

FIG.  97.— Yellow  perch  (Perca  flavescens).   df,  dorsal  fins  ;  pc,  pectoral  fin  ;  pf,  pelvic 
fin  ;  v,  ventral  fin. 

aiding  the  tail  in  changing  the  course  from  side  to  side ; 
or  they  may  be  used  to  support  the  animal  as  it  rests  upon 
the  bottom  in  wait  for  food  ;  and,  finally,  they  may  serve  to 
keep  the  body  suspended  at  a  definite  point. 

In  addition  to  an  internal  skeleton  the  bass  or  perch, 
like  the  greater  number  of  fishes,  is  more  or  less  enclosed 
and  protected  by  an  external  one,  consisting  of  a  beautifully 
arranged  series  of  overlapping  scales,  which  afford  protec- 
tion to  the  underlying  organs,  and  at  the  same  time  admit 
of  great  freedom  of  movement.  These  usually  consist  of  a 
horny  substance,  to  which  lime  is  sometimes  added,  and 
are  peculiar  modifications  of  the  skin,  something  like  the 
feathers,  nails,  and  hoofs  of  higher  forms. 

147.  The  air-bladder.— Xaturally  a  fish's  body  is  heavier 
than  the  water  in  which  it  lives,  and  there  are  reasons  for 
thinking  that  the  air-bladder  (Fig.  106,  a.U.)  acts  in  the 


156  ANIMAL  FORMS 

bass  and  perch  and  many  other  fishes  as  a  float  to  enable 
them,  without  much  effort,  to  remain  suspended  at  a  defi- 
nite level.  By  compressing  this  sac,  partly  by  its  own  mus- 
cles and  partly  by  those  of  the  body-wall,  the  bulk  of  the 
fish  is  made  less,  and  it  sinks ;  upon  the  relaxation  of  these 
same  muscles  the  body  expands  and  rises  again.  Deep-sea 
fishes,  when  brought  to  the  surface,  where  the  pressure  is 
relatively  slight,  are  found  with  their  air-bladders  so  dis- 
tended that  they  can  not  sink  again,  and  the  float  of  surface 
fishes  would  be  as  useless  if  they  were  to  be  carried  into  the 
depths  below,  so  that  such  fishes  are  compelled  to  keep 
within  tolerably  definite  limits  of  depth.  Morphologically 
considered,  the  air-bladder  is  a  modified  or  degenerate  lung, 
and  in  many  fishes  it  is  lost  altogether. 

148.  Respiration. — Looking  down  the  throat  of  the  perch, 
or  any  other  fish,  a  series  of  slits  (the  gill-openings),  usually 
four  or  five  in  number,  may  be  seen  on  each  side  communi- 
cating with  the  exterior.     In  the  sharks  these  outer  open- 
ings are  readily  seen,  but  in  the  bony  fishes  they  open  into 
a  chamber  on  each  side  of  the  head,  covered  by  a  bony  plate 
or  gill-cover  that  is  open  behind.     On  raising  these  flaps 
the  gills  may  be  seen  composed  of  great  numbers  of  bright- 
red  filaments  attached  to  the  bars  between  each  slit.     Dur- 
ing life  the  fish  may  be  seen  to  open  its  mouth  at  regular 
intervals,  and,  after  gulping  in  a  quantity  of  water,  to  close 
it  again,  contracting  the  sides  of  the  throat  to  force  it  out 
of  the  gill-openings  and  over  the  gill-filaments  to  the  exte- 
rior.    During  this  process  the  blood  traversing  the  excess- 
ively thin  filaments  extracts  the  oxygen  from  the  water  and 
carries  it  to  other  parts  of  the  body. 

With  this  information,  let  us  return  to  the  study  of  the 
three  classes  of  fishes. 

149.  The  lancelet  (Branchiostoma).— The  lancelet,  some- 
times called  amphioxus  (Fig.  98),  the  type  of  the  class  Lepto- 
cardii,  is  a  little  creature,  half  an  inch  to  four  inches  long,  in 
the  different  species,  transparent  and  colorless,  living  in  the 


THE  FISHES  157 

sand  in  warm  seas,  the  nine  species  known  being  found  in 
as  many  different  regions.  A  lancelet  may  be  regarded  as 
a  vertebrate  reduced  to  its  lowest  terms.  Instead  of  a 
jointed  back-bone,  it  has  a  cartilaginous  notochord,  running 
from  the  head  to  the  tail.  A  nervous  cord  lies  above  it, 
enclosed  in  a  membranous  sheath.  No  skull  is  present,  and 
the  nerve-cord  does  not  swell  into  a  brain.  There  are  no 
eyes  and  no  scales.  The  mouth  is  a  vertical  slit,  without 
jaws.  There  is  no  trace  of  the  shoulder-girdle  (shoulder- 
blade  and  collar-bone)  or  pelvis  (hip-bone)  from  which 


FIG.  98.— The  California  lancelet  (Bramhiostoma  californiense).    Twice  the  natural 
size,    g,  gills  ;  I,  liver  ;  m,  mouth  ;  n,  nerve-cord  ;  nc,  notochord. 

spring  the  paired  fins,  which,  in  true  fishes,  correspond  to 
arms  and  legs.  The  circulatory  system  is  fish -like,  but  there 
is  no  heart,  the  blood  being  driven  about  by  the  contraction 
of  the  walls  of  the  vessels.  Along  the  edge  of  the  back  and 
tail  is  a  rudimentary  fin,  made  of  fin-rays  connected  by  mem- 
brane. In  the  character  and  arrangement  of  its  organs  the 
lancelet  is  certainly  like  a  fish,  but  in  degree  of  develop- 
ment it  differs  more  from  the  lowest  fish  than  the  fish  does 
from  a  mammal. 

150.  Lampreys  (or  Cyclostomes). — The  class  of  lampreys 
stands  next  in  development  (Fig.  99).  The  notochord  gives 
way  anteriorly  to  a  cartilaginous  skull,  in  which  is  con- 
tained the  brain,  of  the  ordinary  fish  type.  There  are  eyes, 
and  the  heart  'is  developed,  and  consists  of  an  auricle  and 
a  ventricle.  As  distinguished  from  the  true  fish,  the  lam- 
preys show  no  trace  whatever  of  limbs  or  of  the  bones 
which  would  support  them.  The  lower  jaw  is  wholly  want- 
ing, the  mouth  being  a  roundish  sucking  disk.  The  fins 


158 


ANIMAL  FORMS 


are  better  developed,  but  of  the  same  structure  as  in  the 
lancelet.  There  is  no  bony  matter  in  the  skeleton,  and 
there  are  no  scales.  The  nasal  opening  is  single  on  the  top 
of  the  front  of  the  head. 

There  are  about  twenty-five  species  in  this  class.     Some 
of  them,  called  lampreys,  ascend  the  streams  from  the  sea 


FIG.  99. — Lampreys. 

in  the  spring  for  the  purpose  of  spawning.  The  young 
undergo  a  metamorphosis,  at  first  being  blind  and  tooth- 
less. The  adults  feed  mostly  on  the  blood  of  fishes,  which 
they  suck  after  scraping  a  hole  in  the  flesh  with  their  rasp- 
like  teeth.  The  others,  called  hag-fishes,  live  in  the  sea 
and  bore  into  the  bodies  of  other  fishes,  whose  muscles  they 
devour.  All  are  slender,  smooth,  and  eel-shaped. 

From  their  structure  and  a  few  fossil  remains  we  sup- 
pose that  these  eel-like  forms  existed  long  ago,  probably  be- 
fore the  more  highly  developed  sharks  and  bony  fishes  made 
their  appearance,  but  it  is  difficult  to  determine  whether 
their  simple  organization  is  of  such  long  standing  or  is  not 
in  part  the  result  of  semiparasitic  habits,  or  a  life  spent 


THE  FISHES  159 

largely  in  burrowing.  Like  the  lancelet  and  other  simple 
chordates,  they  are  of  the  greatest  interest  to  the  zoologist 
who  gains  from  them  some  idea  of  the  lowly  vertebrate 
forms  that  peopled  the  earth  long  ago. 

151.  True  fishes, — The  third  class,  Pisces  or  true  fishes, 
to  which  the  shark  as  well  as  the  bass  and  perch  belong  has 
a  well-developed  skeleton,  skull,  and  brain.     The  lower  jaw 
is  developed,  forming  a  distinct  mouth,  and  there  is  at  least 
a  shoulder-girdle  and  pelvis ;  although  the  fins  these  should 
bear  are  not  always  developed,  the  general  traits  are  those 
we  associate  with  the  fish.     Of  the  true  fishes,  there  are 
again  several  strongly  marked  groups,  usually  called  sub- 
classes.    Of  these,  three  chiefly  interest  us. 

152.  The  sharks  and  skates. — Very  early  in  the  life  of 
the  sharks  (Fig.  100)  and  skates  (Selachii  or  Elasmobranchii) 


' 


FIG.  100.— Young  shark  (Galeus  zyopterus).    One-seventh  natural  size. 

a  notochord  appears,  similar  to  that  in  the  lancelet  and  the 
lampreys.  As  growth  proceeds  its  sheath  becomes  broken 
up  into  a  series  of  cartilaginous  rings,  which  thus  appear 
like  spools  strung  on  a  cord.  As  the  fish  grows  older  these 
"  spools  "  or  vertebrae  grow  solid,  cutting  the  notochord  into 
little  disks,  and  great  flexibility  is  thus  secured.  Cartilagi- 
nous appendages  also  grow  up  and  cover  the  spinal  nerve- 
cord  lying  above,  and  give  strength  to  the  unpaired  fins ; 
the  paired  fins  also  have  their  supports.  The  shoulder- 
33 


160  ANIMAL  FORMS 

girdle  is  placed  behind  the  skull,  leaving  room  for  a  distinct 
neck  ;  strong  bars  of  cartilage  bear  the  gills ;  others  form  jaws 
to  carry  the  teeth ;  and  a  complex  skull  protects  the  brain 
and  sense-organs,  which  are  of  a  relatively  high  state  of  devel- 
opment. Throughout  life  the  skeleton  is  of  cartilage,  with 
perhaps  here  and  there  a  little  bone  where  greater  strength 
is  required.  Besides  these,  there  are  numerous  minor 
characters  which  the  student  will  readily  find  for  himself. 

The  sharks  and  skates  or  rays  live  chiefly  in  the  sea, 
and  some  reach  an  enormous  size,  the  largest  of  all  fishes. 
Some  are  very  ferocious  and  voracious ;  others  are  very  mild 
and  weak,  and  the  development  of  teeth  is  in  direct  pro- 
portion to  their  voracity  of  habit.  In  earlier  geologic  times 
there  were  many  more  species  of  them  than  now  exist. 

153.  The  lung-fishes. — The  lung-fishes  (Dipnoi)  are  pe- 
culiar forms  living  in  some  of  the  rivers  of  Australia  and 
the  tropical  regions  of  Africa  and  South  America.    In  these 
the  air-bladder  is  developed  as  a  perfect  lung.     During  the 
wet  season  they  breathe  like  other  fishes  by  means  of  gills, 
but  as  the  rivers  dry  up  they  burrow  into  the  wet  mud  and 
breathe  by  means  of  lungs  which  are  spongy  sacs  of  which 
the  air-bladder  of  other  fishes  is  a  degenerate  representative. 
As  we  shall  see,  they  resemble  in  this  respect  the  tadpoles 
and  some  adult  Amphibia  (frogs  and  salamanders).     The 
paired  fins  are  also  peculiar  in  structure,  having  an  elongate 
jointed  axis,  with  a  fringe  of  rays  along  its  length,  a  struc- 
ture almost  as  much  like  that  of  the  limbs  of  a  frog  as  that 
of  a  fish's  fin.     In  fact  the  Dipnoi  must  be  regarded  as  an 
ancestral  type,  an  ally  of  the  generalized  form  from  which 
Amphibia  and  bony  fishes  have  descended.     Only  four  liv- 
ing species  of  dipnoans  are  known,  but  great  numbers  of 
fossil  species  are  found  in  the  rocks. 

154.  The  bony  fishes   (Teleostei).— The  bony  fishes,  or 
Teleosts,  are  distinguished  by  the  bony  skeleton,  the  sym- 
metrical tail,  and  by  the  development  of  the  air-bladder  as 
a  more  or  less  completely  closed  sac,  useless  in  respiration. 


THE  FISHES  161 

Often  this  organ  is  altogether  wanting,  as  in  the  common 
mackerel.  About  ten  thousand  kinds  of  bony  fishes  are 
known.  The  species  swarm  in  every  sea,  lake,  or  river 
throughout  the  earth,  and  some  form  or  another  among 
them  is  familiar  to  every  boy  in  the  land.  These  fishes  are 
divided  into  about  two  hundred  families,  and  these  may  be 
arranged  in  fifteen  to  twenty  orders.  As  these  are  mostly 
distinguished  by  features  of  the  skeleton,  we  need  not  name 
them  here.  In  Jordan  and  Evermann's  Fishes  of  North  and 
Middle  America,  as  well  as  in  various  other  books,  the  stu- 
dent of  fishes  can  find  the  characters  by  which  orders  may 
be  distinguished. 

155.  Sturgeons  and  garpikes  (Ganoidea). — While  the  great 
majority  of  the  typical  fishes  possess  a  bony  skeleton,  there 
are  a  few  quaint  types — the  ganoid  fishes,  such  as  the  stur- 
geons (Fig.  101)  and  garpikes — in  which  it  is  cartilaginous  or 
partly  bony.  In  past  ages  these  were  probably  the  highest 
type  of  fishes,  and  from  their  fossil  remains  we  may  con- 
clude that  they  flourished  in  vast  numbers ;  but  at  present 
they  are  almost  extinct.  In  this  country  the  ganoids  are 
represented  by  several  species,  the  best  known  being  the 
sturgeons  which  inhabit  the  Great  Lakes,  the  Mississippi, 
and  its  tributaries;  while  on  the  East  coast  the  common 
sturgeon  (Acipenser  sturio)  often  leaves  the  sea  and  ascends 
rivers.  They  are  the  largest  fishes  found  in  fresh  water, 
attaining  a  length  of  ten  or  twelve  feet,  and  a  weight  of 
five  hundred  pounds.  Their  food  consists  of  small  plants 
and  animals,  which  they  suck  in  through  their  tube-like 
mouth.  The  garpikes  live  in  the  larger  lakes  and  rivers 
throughout  the  East  and  Mississippi  Valley.  Their  bodies, 
from  three  to  ten  feet  in  length,  according  to  the  species, 
are  covered  with  comparatively  large  regularly  arranged 
square  scales,  and  the  upper  jaw  is  elongated  to  form  a 
kind  of  beak,  abundantly  supplied  with  teeth.  They  are 
carnivorous,  voracious  fishes,  working  great  havoc  among 
the  more  defenseless  food-fishes. 


THE  FISHES  163 

156.  The  catfishes.— Among  the  lowest  bony  fishes  we 
may  place  the  great  group  to  which  almost  all  fresh-water 
fishes  belong.    In  this  group  the  four  vertebrae  situated  next 
the  head  are  firmly  united,  and  by  means  of  certain  small 
lever-like  bones  a  connection  is  formed  between  the  air-blad- 
der and  the  ear  of  the  fish,  which  is  sunk  deep  in  the  skull. 
The   air-bladder  thus   becomes  a  sounding   organ  in  the 
function  of  hearing.     The  family  of  catfishes  possesses  this 
structure,  and  the  student  should  look  for  it  in  the  first  one 
he  catches.     The  catfishes  are  remarkable  for  the  long  feel- 
ers about  the  mouth,  with  which  they  pick  their  way  on  the 
bottom  of  a  pond.     There  are  many  kinds  the  world  over. 
The  small  ones  are  known  as  horned  pout  or  bullhead.     In 
these  the  dorsal  and  pectoral  fins  are  armed  each  with  a 
strong,  sharp  spine,  which  is  set.  stiff  when  the  fish  is  dis- 
turbed, and  makes  them  very  troublesome  to  handle.     The 
catfishes  have  no  scales. 

157.  The  carp-like  fishes.— The  still  greater  carp  family 
includes  all  the  carp,  dace,  minnows,  and  chubs.     They 
have  the  air-bladder  joined  to  the  ear,  just  like  the  catfish, 
but  they  lack  the  long  feelers  and  the  fin  spines,  while  the 
soft  body  is  covered  with  scales,  and  there  are  no  teeth  in 
the  mouth.    In  the  throat  are  a  few  very  large  teeth,  which 
the  ingenious  boy  should  find.     In  the  sucker  family  these 
throat  teeth  are  like  the  teeth  of  a  comb,  and  the  mouth  is 
fitted  for  sucking  small  objects  on  the  river  bottom. 

158.  The  eels. — In  the  great  order  of  eels  the  body  is 
long  and  slim,  scaleless,  or  nearly  so,  with  no  ventral  fins. 
The  shoulder-girdle  has  slipped  back  from  the  head,  so  as 
to  leave  a  distinct  neck,  while  ordinary  fishes  have  none- 
Of  eels  there  are  very  many  kinds — some  large  and  fierce, 
some  small  as  an  earthworm ;  and  one  kind  comes  into  fresh 
water. 

159.  Herring  and  salmon. — In  the  great  order  which  in- 
cludes the  herring  and  salmon  the  vertebras  are  all  alike, 
the  ventral  fins  far  from  the  head,  and  the  scales  smooth  to 


164  ANIMAL  FORMS 

the  touch.  The  herring  and  shad  are  examples,  as  also  the 
salmon  and  trout.  Some  live  in  the  great  depths  of  the 
sea,  even  five  miles  below  the  surface.  These  are  very  soft 
in  body,  being  under  tremendous  pressure.  They  are  inky 
black — for  the  sea  at  that  depth  seems  black  as  ink — and 
most  of  them  have  luminous  spots  which  give  them  light 
in  the  darkness.  Some  species  have  the  forehead  luminous, 
like  the  headlight  of  an  engine.  Most  of  these  deep  sea 
fishes  are  very  voracious,  for  there  is  nothing  for  them  to 
feed  on  save  their  neighbors. 

160.  The  pike,  sticklebacks,  etc.— Several  small   orders 
stand  between  these  soft-rayed,  smooth-scaled  fishes  and 


c 

FIG.  108.— The  blindfish  and  its  parentage.  A,  Dismal  Swamp  fish  (Chologaster 
amtus\  the  ancestor  of  (B)  Agassiz's  cave  fish  (Chologaster  agassizi)  and  (C) 
cave  blindflsh  (Typhlichthys  subterraneus). 

the  form,  like  the  perch  and  bass,  which  has  many  spines  in 
the  dorsal  fin.  Among  these  transitional  forms  is  the  pike 
(Fig.  103) — long,  slender,  circumspect,  and  voracious,  lying 
in  wait  under  a  lily-pad ;  the  blindfish,  which  lost  its  eyes 
through  long  living  in  the  streams  of  the  great  caves ;  the 
stickleback,  small,  wiry,  malicious,  and  destructive,  steal- 
ing the  eggs  and  nibbling  the  fins  of  any  larger  fish; 
the  sea-horse,  often  clinging  with  its  tail  to  floating 


166  ANIMAL  FORMS 

seaweed,  the  male  carrying  the  eggs  about  in  his  pocket 
until  they  hatch ;  the  mullet,  stupid,  blundering,  feeding 
on  minute  plants,  crushing  them  in  a  gizzard  like  that  of 
a  hen,  but  withal  having  soft  flesh,  good  for  the  table ;  the 
flying-fishes,  which  sail  through  the  air  with  great  swiftness 
to  escape  their  enemies. 

161.  The  spiny-rayed  fishes. — In  the  group  of  spiny- 
rayed  fishes  the  ventral  fins  are  brought  forward  and  joined 
to  the  shoulder-girdle.  The  scales  are  generally  rough  to 
the  touch,  and  the  head  is  usually  roughened  also.  There 
are  many  in  every  sea,  ranging  in  size  from  the  Everglade 
perch  of  Florida,  an  inch  long,  to  the  swordfish,  which  is 
thirty.  These  are  the  most  specialized,  the  most  fish-like 
of  all  the  fishes.  Leading  families  are  the  perch,  in  the 
fresh  waters,  the  common  yellow  perch,  familiar  to  all  boys 
in  the  Northeastern  States ;  the  darters,  which  are  dwarf 
perches,  beautifully  colored  and  gracefully  formed,  living 
on  the  bottoms  of  swift  rivers ;  the  sunfishes,  with  broad 
bodies  and  shining  scales,  thriving  and  nest-building  in 
the  quiet  eddies ;  the  sea-bass  of  many  kinds,  all  valued  for 
the  table ;  the  mackerel  tribe,  mostly  swimming  in  great 
schools  from  shore  to  shore.  After  these  come  the  multi- 
tude of  snappers,  grunts,  weakfishes,  bluefishes,  rose-fishes, 
valued  as  food.  Then  follow  the  gurnards,  with  bony 
heads;  the  sculpins,  with  heads  armed  with  thorns,  the 
small  ones  in  the  rivers  most  destructive  to  the  eggs  of 
trout ;  and  at  the  end  of  the  long  series  a  few  families  in 
which  the  spines  once  developed  are  lost  again,  and  the 
fins  have  only  soft  and  jointed  rays.  It  is  a  curious  law  of 
development  that  when  a  structure  is  once  highly  special- 
ized it  may  lose  its  usefulness,  at  which  point  degeneration 
at  once  sets  in.  Among  fishes  of  this  type  are  the  cod- 
fishes, with  spindle-shaped  bodies,  and  the  flounders,  with 
flat  bodies.  The  flounders  lie  on  the  sand  with  one  side 
down,  and  the  head  is  so  twisted  that  the  eyes  come  out  to- 
gether on  the  side  that  lies  uppermost.  This  side  is  col- 


168  ANIMAL  FORMS 

ored  like  the  bottom — sand  colored  or  brown  or  black — and 
the  under  side  is  white.  When  the  flounder  is  first  hatched, 
the  eyes  are  on  each  side  of  the  head,  and  the  animal 
swims  upright  in  the  water  like  other  fishes.  But  it  soon 
rests  on  the  bottom ;  it  turns  to  one  side,  and  as  the  body 
is  turned  over  the  lower  eye  begins  to  move  over  to  the 
other  side.  Finally,  we  may  close  the  series  with  the  an- 
glers (Fig.  105),  in  which  the  first  dorsal  spine  is  trans- 
formed into  a  sort  of  fishing-pole  with  a  bait  at  the  end, 
which  may  sometimes  serve  to  lure  the  little  fishes,  which  are 
soon  swallowed  when  once  in  reach  of  the  capacious  mouth. 

162.  Internal  anatomy. — A  few  fishes  are  vegetarians,  but 
the  greater  number  are  carnivorous.  Some  swallow  large 
quantities  of  sand  of  the  sea-bottom  and  absorb  from  it  the 
small  organisms  living  there.  Others  are  provided  with 
beaks  for  nipping  off  corals  and  tube-dwelling  worms.  Huge 
plate-like  teeth  enable  others  to  crush  mollusks,  sea-urchins, 
and  crabs,  and  many  are  adapted  for  preying  upon  other 
fishes.  The  latter  are  often  able  to  escape,  owing  to  the 
presence  of  numerous  spines,  sometimes  supplied  with 
poison-glands;  or  their  colors  are  protective,  and  a  vast 
number  of  devices  are  present  which  enable  them  with 
some  degree  of  surety  to  escape  their  enemies  and  capture 
food. 

Usually,  without  mastication,  the  food  passes  into  the 
digestive  tract  (Fig.  106),  which  in  the  main  resembles  that 
of  the  squirrel,  but  varies  considerably  according  to  the 
nature  of  the  food  it  is  required  to  absorb.  As  in  other 
animals,  it  is  usually  longer  in  the  vegetable  feeders.  In 
most  fishes  the  walls  of  the  canal  are  pushed  out  at  the 
junction  of  the  stomach  and  intestine,  to  form  numerous 
processes  like  so  many  glove-fingers  (the  pyloric  coeca,  Fig 
106,  pyx.),  which  probably  serve  to  increase  the  absorptive 
surface.  The  same  result  is  obtained  in  other  ways,  chiefly 
by  numerous  folds  of  the  lining  of  the  canal. 

The  blood-system  is  much  more  complex  in  the  fishes 


THE  FISHES 


169 


than  in  any  of  the  invertebrates.  It  also  differs  in  its  gen- 
eral plan  from  that  of  most  adult  vertebrates,  owing  to  the 
peculiar  method  of  respiration.  In  almost  every  case  the 


FIG.  105.— Angler  or  frogfish  (Lophius  piscatorius}.    One-tenth  natural  size.— After 

BASKETT. 

vessels  returning  from  all  parts  of  the  body  unite  into  one 
vein  leading  into  the  heart,  which  consists  of  only  one 
auricle  and  ventricle  (Fig.  106).  From  the  heart  the  blood 


170  ANIMAL  FORMS 

is  forced  through  the  gills,  with  all  their  delicate  filaments, 
and  now,  laden  with  oxygen  and  nutritious  substances  al- 
ready absorbed  from  the  coats  of  the  digestive  tract,  it 


FIG.  106.— Dissection  of  a  bony  fish,  the  trout  (Scdmo).  a.bl.,  air-bladder  ;  an.,  anal 
opening;  at/.,  auricle;  gl.st.,  gills;  gul.,  esophagus;  int.,  intestine;  kd.,  kidney ; 
lr.,  liver;  l.ov.,  ovary;  opt.l.,  brain  ;  py.c.,  pyloric 'coeca  ;  sp.c.,  spinal  cord  ;  spl., 
spleen  ;  st.,  stomach  ;  v.,  ventricle. 

travels  on  to  all  parts  of  the  body,  continually  unloading 
its  cargo  in  needy  districts  and  waste  matters  in  the  kid- 
neys before  returning  once  more  to  the  heart. 

163.  The  senses  of  fishes.— The  habits  of  fishes  indicate 
that  they  know  considerable  of  what  is  going  on  in  the 
outside  world,  and  their  well-developed  sense-organs  show 
the  degree  of  their  sensitiveness.  A  share  of  this  informa- 
tion comes  through  the  sense  of  touch,  which  is  distributed 
all  over  the  surface  of  the  body,  chiefly  in  the  more  ex- 
posed regions  sometimes  especially  provided  with  fleshy 
feelers,  like  those  on  the  chin  of  the  catfish. 

The  sense  of  smell  appears  to  be  fairly  developed,  as  is 
that  of  hearing ;  but  there  is  no  evidence  of  a  sense  of  taste. 
A  few  fishes  chew  their  food,  and  may  possibly  taste  it,  but 
there  are  others  that  swallow  it  whole,  and  in  all  there  are 
relatively  a  few  nerves  going  to  the  tongue  or  floor  of  the 
mouth. 


THE  FISHES 

The  eyes  of  most  fishes  are  highly  developed,  and  are  of 
the  greatest  use  at  all  times.  Exceptions  to  the  rule  are 
found  in  certain  species  which  live  in  caves  or  in  the  dark 
abysses  of  the  ocean.  In  some  of  these  the  eyes  have  dis- 
appeared almost  completely,  and  the  sense  of  touch  be- 
comes correspondingly  more  acute  ;  in  other  deep-sea  forms 
they  have  grown  to  a  large  size,  enabling  them  to  distin- 
guish objects  in  the  gloom,  like  the  owls  and  other  noc- 
turnal animals.  Embedded  in  the  skin  of  some  of  these 
deep-sea  fishes,  and  certain  nocturnal  ones,  are  peculiar 
spots,  composed  of  a  glandular  substance,  which  produces 
a  bright  glow  like  that  of  the  fireflies.  These  may  be  located 
on  the  head  or  arranged  in  patterns  over  various  parts  of 
the  body,  and  may  serve  to  light  the  fish  on  its  way  and 
enable  it  to  see  its  food  to  better  advantage,  or  it  may  act 
as  a  lure  to  many  fishes  that  become  victims  to  their  own 
curiosity.  In  those  fishes  which  are  active  most  of  the 
time  the  eyes  are  located  on  the  sides  of  the  head,  and  in 
those  which  remain  at  or  near  the  bottom  they  are  turned 
toward  the  top ;  in  every  case  where  they  can  be  used  to 
the  best  advantage. 

164.  Breeding  habits. — Fishes  usually  lay  their  eggs  in 
spring,  the  salmon-trout  and  others  in  the  winter.  In  breed- 
ing-time the  males  often  grow  resplendent,  and  may  engage 
in  struggles  for  their  respective  mates.  In  others  this 
ceremony  is  performed  without  show  of  hostility.  Some 
make  nests,  while  others  lay  their  eggs  loosely  in  the  water. 

In  all  the  salmon  family  the  young  fishes  are  born  in 
the  colder  fresh-water  rivers,  and  later  make  their  way  into 
the  sea,  where  they  spend  the  greater  part  of  their  lives. 
When  the  time  comes  for  them  to  lay  their  eggs  they 
migrate  in  great  companies,  and  make  their  way  hundreds, 
perhaps  thousands,  of  miles  to  the  rivers  in  which  they 
spent  their  youth.  Up  these  streams  they  rush  in  crowds, 
leaping  waterfalls  and  rapids,  and,  dashed  and  battered  on 
the  rocks,  many,  and  in  some  species  all,  die  from  injuries 


172  ANIMAL  FORMS 

or  exhaustion  after  the  breeding  season  is  passed.  The 
eggs,  like  those  of  the  chubs,  suckers,  sunfishes,  and  cat- 
fishes,  are  usually  buried  in  shallow  holes  in  the  sand,  and 
the  males  of  most  fishes  keep  a  faithful  watch  over  the 
young  until  they  are  able  to  live  in  safety.  In  some  of 
the  sticklebacks  and  several  marine  species  elaborate  nests 
are  composed  of  grass  or  seaweeds ;  some  of  the  catfishes 
carry  the  eggs  until  they  hatch  in  their  mouths  or  else  in 
folds  of  spongy  skin  on  the  under  side  of  the  body  ;  in  the 
pipefishes  and  sea-horses  a  slender  sac  along  the  lower  sur- 
face of  the  male  acts  as  a  brood-pouch,  in  which  the  female 
places  the  eggs  to  remain  until  developed ;  and  some  fishes, 
such  as  the  surf-fishes  and  a  number  of  the  sharks,  bring 
forth  their  young  alive.  On  the  other  hand,  the  young  of 
many  of  the  herrings,  salmon,  cod,  perch,  and  numerous 
other  fishes  are  abandoned  at  their  birth,  and  fall  a  prey  to 
many  animals,  even  their  parents  often  included. 

In  the  former  cases,  where  the  young  are  protected,  only 
a  relatively  few  eggs  are  produced :  where  they  are  aban- 
doned the  female  often  lays  many  millions.  In  every  case 
the  number  of  eggs  is  in  direct  relation  to  the  chances  the 
young  have  of  reaching  maturity,  a  few  out  of  each  brood 
surviving  to  perpetuate  the  race. 

165.  Development  and  past  history.— The  eggs  of  the 
higher  bony  fishes  are  usually  small  (one-tenth  to  one- third 
of  an  inch  in  diameter),  and  the  young  when  they  hatch 
are  accordingly  little ;  in  the  sharks  the  eggs  are  larger, 
the  size  of  a  hen's  egg  or  even  larger,  and  the  young  when 
born  are  relatively  large  and  powerful.  These  differences, 
however,  do  not  greatly  affect  the  early  development,  for 
in  every  case  the  head  and  then  the  trunk  soon  become 
formed,  gills  arise,  the  nervous  system  appears,  which  is 
invariably  supported  by  a  skeleton  in  the  form  of  a  gristly 
rod— the  notochord.  In  the  lower  forms  of  fishes  this  per- 
sists throughout  life ;  but  in  the  sharks  and  skates  it"  be- 
comes replaced  in  the  adult  by  another  and  higher  type  of 


THE  FISHES  173 

skeleton,  which  is  much  more  specialized  with  the  hony 
fishes. 

Those  who  study  the  fossils  on  the  rocks  tell  us  that 
the  first  fishes  were  very  simple,  and  many  believe  that 
their  skeleton,  like  that  of  the  little  growing  fish,  consisted 
only  of  a  notochord.  Many  of  these  old  forms  died  out 
long  ago,  while  others  gradually  changed  in  one  way  and 
another  to  adapt  themselves  to  their  surroundings,  the  con- 
stant need  of  adaptation  having  resulted  in  the  multitude 
of  present-day  types.  Some,  such  as  the  lamprey,  have 
probably  changed  relatively  only  to  a  slight  extent ;  others, 
like  the  sharks  and  skates,  are  much  more  altered;  and 
the  bony  fishes  are  far  from  their  original  low  estate, 
though  their  development  has  been  rather  toward  a  greater 
specialization  for  aquatic  life  than  an  advance  upward. 
The  little  fish  in  its  growth  from  the  egg  thus  repeats  the 
history  of  its  ancestral  development;  but  as  though  in 
haste  to  reach  the  adult  condition,  it  omits  many  impor- 
tant details.  Moreover,  the  record  in  the  Tocks  is  not 
complete,  and  we  have  many  things  yet  to  learn  of  the 
ancient  fishes  and  their  development  from  age  to  age  to 
the  present  day. 


CHAPTEK  XV 

THE    AMPHIBIANS 

IN  many  respects  the  amphibians — toads,  frogs,  and  sala- 
manders—resemble the  fishes,  especially  the  lung-fishes 
(Dipnoi).  The  modern  amphibians  are  essentially  fishes 
in  their  early  life,  but  in  developing  legs  and  otherwise 
changing  their  bodily  form  they  become  adapted  for  a  life 
on  land  under  conditions  differing  from  those  of  the  fishes. 
Judging  from  this  class  of  facts,  we  may  assume  that  fish- 
like  ancestors,  by  the  development  of  the  lungs,  became 
fitted  for  a  life  on  land,  and  that  from  these  the  amphib- 
ians of  our  times  have  been  derived. 

166.  Development. — The  eggs  of  the  Amphibia  are  laid 
during  the  spring  months  in  fresh-water  streams  and  ponds. 
They  are  globular,  about  as  large  as  shot,  and  are  embedded 
in  a  gelatinous  envelope  (Fig.  107).  They  are  either  de- 
posited singly  or  in  clumps,  or  festooned  in  long  strings  over 
the  water-weeds.  During  the  next  few  days  development 
proceeds  rapidly  under  favorable  conditions,  resulting  in  an 
elongated  body  with  simple  head  and  tail.  In  this  condition 
they  are  hatched  as  tadpoles.  As  yet  they  are  blind  and 
mouthless,  but  lips  and  horny  jaws  soon  appear,  along  with 
highly  developed  eyes,  ears,  and  nose.  External  fluffy  gills 
arise  on  the  sides  of  the  head,  and  slits  form  in  the  walls  of 
the  throat,  between  which  gills  are  attached,  and  over  which 
folds  of  skin  develop,  as  in  the  fishes.  A  fin-fold  like  that 
of  the  lancelet  or  lamprey  appears  on  the  tail.  The  brain 
and  spinal  cord,  extending  along  the  line  of  the  back,  are 
supported  by  a  gristly  notochord,  and  complete  and  com- 
174 


THE  AMPHIBIANS 


175 


plex  internal  organs  adapt  the  animal  to  a  free-swimming 
existence  for  days  to  come. 

The  tadpole  is  now,  to  all  intents  and  purposes,  a  fish — 
a  fact  most  clearly  recognized  in  its  form,  method  of  loco- 


Fio.  107.— Metamorphosis  of  the  toad.— Partly  after  GAGE,  from  Animal  Life. 

motion,  the  arrangement  of  the  gills,  and  the  general  plan 
of  the  circulatory  system. 

167.  Further  growth. — In  the  course  of  the  next  few 
weeks  hind  limbs  develop  beneath  the  skin,  through  which 
they  finally  protrude.  In  the  same  manner,  fore  limbs  arise 
at  a  later  date.  In  position  these  organs  are  like  the  paired 
fins  of  fishes,  but  they  are  intended  for  crawling  or  leaping 
on  land,  and  are  modified  in  accordance  with  this  need.  As 
in  the  higher  vertebrates,  the  limbs  develop  as  arms  and 
legs,  with  long  fingers  and  toes,  between  which  are  stretched 
webs  of  skin,  which  serve  in  swimming. 
34 


176  ANIMAL  FORMS 

In  the  meantime  large  internal  changes  are  also  taking 
place.  The  wall  of  the  esophagus  has  gradually  pouched 
out  to  form  the  lungs.  They  are  richly  supplied  with  blood- 
vessels, closely  resembling  in  their  general  features  the 
lungs  of  the  lung-fishes.  The  animal  now  rises  to  the  sur- 
face occasionally  to  gulp  in  air,  and  it  also  continues  to 
breathe  by  means  of  gills.  At  this  stage  of  its  existence, 
therefore,  the  larva  is  amphibious  (two-living),  and  we  have 
the  interesting  example  of  an  animal  extracting  oxygen 
from  both  the  water  and  the  air.  The  diet  of  the  tadpole 
at  this  time  changes  from  vegetable  to  animal  substances, 
and  horny  teeth  give  way  to  the  small  teeth  of  the  frog, 
and  the  digestive  system  undergoes  an  entire  remodeling 
to  adapt  it  to  its  new  duties.  The  young  amphibian — 
whether  frog,  toad,  or  salamander— is  now  a  four-legged 
creature,  with  well-developed  head  and  tail,  with  lungs  and 
gills,  though  the  latter  are  usually  fast  disappearing,  and  is 
rapidly  assuming  those  characters  which  will  fit  it  for  a 
terrestrial  or  semiaquatic  existence. 

168.  The  salamanders. — The  changes  which  now  ensue  in 
such  a  larva  in  reaching  the  adult  condition  are  relatively 
slight  in  the  lower  salamanders.  The  external  gills  often 
persist  (Fig.  110),  the  lungs  are  also  functional,  and  the 
changes  are  largely  those  of  increase  of  size.  In  the  larger 
number  of  species  the  gills  disappear  more  or  less  com- 
pletely (Fig.  108),  such  species  often  abandoning  the  water 
for  homes  in  damp  soil  or  under  stones  and  logs,  returning 
to  it  only  when  the  time  comes  for  their  eggs  to  be  laid. 
The  limbs  are  always  relatively  weak,  never  supporting  the 
body  from  the  ground,  but  serving  in  a  clumsy  way  to  push 
it  from  place  to  place.  In  the  aquatic  forms  the  tail  con- 
tinues to  serve  as  a  swimming  organ.  In  some  species  the 
hind  legs  become  rudimentary,  or  even  entirely  lacking. 
A  still  further  modification  occurs  in  a  few  burrowing  spe- 
cies, which  move  by  wrigglings  of  the  body,  and  are  with- 
out either  pairs  of  legs. 


THE  AMPHIBIANS  177 

In  geological  times  many  of  the  salamanders  were  of 
great  size,  several  feet  in  length,  and  some  were  enclosed 
in  an  armor  consisting  of  bony  plates.  All  now  living  have 
the  skin  naked,  and  with  the  exception  of  the  giant  species 
of  Japan,  three  feet  in  length,  and  a  few  similar  forms  in 
America,  the  modern  representatives  are  comparatively 


FIG.  108.— Blunt-nosed  salamander  (Amblystoma  opacum).     Photograph  by  W.  H. 

FISHER. 

feeble  and  measure  their  length  by  inches.  Only  a  few,  on 
account  of  their  bright  colors,  are  particularly  attractive, 
while  the  others  are  usually  shunned  and  considered  re- 
pulsive, chiefly  because  of  their  supposed  poisonous  char- 
acter, though  in  reality  few  animals  are  more  harmless. 

169.  Tailless  forms. — In  the  frogs  and  toads  the  meta- 
morphosis which  the  young  undergo  is  almost  as  profound 
as  that  which  takes  place  with  the  insects.  The  gills,  to- 
gether with  their  blood-vessels,  disappear  completely.  The 
tail,  with  its  muscles,  nerve-supply,  and  skeleton,  is  ab- 
sorbed. The  cartilaginous  notochord  gives  way  to  a  jointed 
back-bone.  A  skull  is  developed ;  numerous  bones  form  in 
the  limbs,  affording  an  attachment  for  the  powerful  muscles 
which  make  the  toad,  and  especially  the  frog,  expert  swim- 


178  ANIMAL  FORMS 

mers  and  leapers,  and  thus  equipped  they  hereafter  lead  a 
wholly  terrestrial  or  seniiaquatic  life. 

170.  Distribution  and  common  forms. — All  the  Amphibia 
are  dependent  upon  moisture.  Almost  all  are  hatched  and 
developed  in  fresh  water,  and  those  which  leave  the  water 
return  to  it  during  the  breeding  season.  So  we  find  repre- 
sentatives of  the  group  all  over  the  world  having  much  the 
same  range  as  the  fresh-water  fishes.  The  great  majority 
of  the  salamanders  are  confined  to  the  northern  hemisphere, 
but  the  toads  and  frogs  are  almost  universally  distributed. 

Among  the  salamanders  in  this  country  only  a  relatively 
few  species  completely  retain  their  external  gills.  This  is 
the  case  with  sirens  and  mud-puppies  or  water-dogs  (Fig. 
110),  which  may  occasionally  be  seen  in  the  clear  waters 
of  our  lakes  and  rivers  crawling  slowly  about  in  search  of 
food,  and  every  now  and  then  rising  to  the  surface  to  gulp 
in  air.  The  remainder  lose  their  gills  more  or  less  com- 
pletely, and  usually  leave  the  water  for  damp  haunts  on 
land.  One  of  the  blunt-nosed  salamanders,  known  as  the 
tiger  salamander  (Amblystoma  tigrinunt),  is  found  in  moist 
localities  in  most  parts  of  the  United  States.  Besides  these 
are  numerous  small  species,  among  them  the  newts  (Die- 
myctylus))  ranging  widely  over  the  United  States,  living 
under  logs  and  stones  and  feeding  upon  the  small  insects 
and  worms  inhabiting  such  situations.  In  several  species 
of  salamanders  the  lungs  disappear  with  age,  and  respira- 
tion is  performed  solely  through  the  surface  of  the  skin. 

The  tailless  amphibians  are  much  more  abundant  and 
familiar  objects  than  the  salamanders,  and  from  the  open- 
ing of  spring  until  late  in  the  fall  they  are  met  with  on 
every  hand.  With  few  exceptions  the  frogs  live  in  or  about 
ponds  and  marshes,  in  which  they  obtain  protection  in 
troublous  times  and  from  which  they  derive  the  store  of 
worms  and  insects  that  serve  as  food.  On  the  other  hand, 
the  tree-frogs,  as  their  name  indicates,  usually  abandon  the 
water  and  repair  to  moist  situations  in  trees  and  other  vege- 


THE  AMPHIBIANS  179 

tation.  Their  shrill,  cricket-like  calls  are  often  heard  in 
the  summer.  The  fingers  and  toes  are  more  or  less  dilated 
into  disks  at  their  tips,  enabling  them  to  climb  with  con- 
siderable facility;  and  they  are  further  adapted  to  their 
surroundings  on  account  of  their  protective  colors.  The 
toads  undergo  their  metamorphosis  while  very  small,  and 
approach  the  water  only  at  the  breeding  season.  During 
the  day  they  remain  concealed  in  holes  and  crevices,  but  at 
the  approach  of  evening  come  out  in  search  of  food. 

171.  Means  of  defense,— The  food  of  the  members  of  this 
group  consists  chiefly  of  small  fishes,  insect  larvae,  snails, 
and  little  crustaceans,  which  are  swallowed  whole.     On  the 
other  hand,  many  Amphibia  prey  on  each  other,  while  most 
of  them  are  eagerly  sought  by  birds  and  fishes.     Some,  as 
the  toads,  stalk  their  food  only  during  the  night-time  or 
depend  upon  their  agility  to  escape  their  enemies.     Others 
are  colored  protectively,  the  markings  of  the  skin  resem- 
bling the  foliage  of  the  earth  upon  which  they  rest,  and  in 
some  species,  as  the  tree-toads,  this  color-pattern  changes 
as  the  animal  shifts  its  position.     A  few  species  are  most 
brilliantly  colored  with  red,  green,  yellow,  or  combinations 
of  these,  in  striking  contrast  to  their  surroundings.     They 
have  apparently  few  enemies,  possibly  because  of  an  un- 
pleasant odor  or  taste,  and  it  has  been  suggested  that  their 
gorgeous  tints  are  danger-signals,  warning  their  would-be 
captors  from  attempting  a  second  time  to  devour  them.   At 
the  same  time  it  is  well  known  that  the  somber-hued  toads 
emit  a   milky  secretion   from   the  warty  protuberance  of 
their  skin  which  is  intensely  bitter,  irritating  to  delicate 
skin,  and  poisonous  to  several  animals. 

172.  Skeleton.— As  in  all  vertebrates,  the  skeleton  of  the 
amphibian  first  arises  as  a  cartilaginous  rod,  the  notochord, 
which  is  afterward   replaced  by  a   jointed  back-bone,  to 
which  the  limbs  are  attached.     The  back-bone  is  anteriorly 
modified  into  a  flat,  usually  complex,  skull.     In  the  sala- 
manders the  number  of  vertebras  is  sometimes  very  large, 


180 


ANIMAL  FORMS 


and  the  body  correspondingly  long  and  snake-like  ;  but  in 
other  cases  parts  of  the  vertebrae  are  reduced  in  number, 
and  the  body  is  rather  short  and  thick.  In  the  frogs  and 
toads  this  reduction  reaches  its  culmination,  for  only  nine 
distinct  vertebrae  are  present,  the  tail  vertebrae,  correspond- 
ing to  those  of  the  salamanders,  being  represented  by  a 
rod-like  bone,  the  urostyle,  made  of  segments  grown  to- 
gether. 

173.  Digestive  and  other  systems. — In  its  main  characters 
the  digestive  tract  of  the  amphibian  (Fig.  109)  resembles 


p.na. 


tng 


FIG.  109.—  Dissection  of  toad  (Bufo).  an.,  anal  opening;  au.,  auricle  ;  bL,  bladder; 
duo.,  duodenum  ;  Ing.,  lung;  lr.,  liver;  pn.,  pancreas  ;  ret.,  rectum  ;  spl,  spleen; 
St.,  stomach ;  v.,  ventricle. 

that  of  the  fishes  and  the  squirrel.  The  mouth  is  usually 
large,  and  the  teeth  are  very  small,  as  in  the  frog  or  sala- 
mander, or  are  lacking  completely,  as  in  the  common  toad. 
In  many  salamanders  the  tongue,  like  that  of  a  fish,  is  fixed 
and  incapable  of  movement.  In  most  of  the  frogs  and 
toads  it  is  attached  to  the  front  of  the  mouth,  leaving  its 
hinder  portion  free,  and  capable  of  being  thrown  over  and 
outward  for  a  considerable  distance.  In  the  throat  region 
gill-clefts  may  persist,  but  they  usually  close  as  the  lungs 
reach  their  development.  The  succeeding  portions  of  the 
canal  are  comparatively  straight  in  the  elongated  forms,  or 


THE  AMPHIBIANS  181 

more  or  less  coiled  in  the  shorter  species.  In  some  cases 
no  well-marked  stomach  exists,  but  ordinarily  the  different 
portions,  as  they  are  shown  in  Fig.  109,  are  well  defined. 

As  noted  above,  the  circulation  in  the  tadpole  is  the 
same  as  in  fishes,  then  lungs  arise,  and  for  a  time  respi- 
ration is  effected  both  by  gills  and  lungs,  and  the  cir- 
culation resembles  in  its  essential  points  that  of  the 
lung-fishes.  This  may  continue  throughout  life,  but  more 
frequently  the  gills  and  their  vessels  disappear,  and  the 
circulation  approaches  that  of  the  reptiles.  In  such  forms 
the  heart  consists  of  two  auricles  and  one  ventricle.  Into 
the  left  auricle  pours  the  pure  blood  from  the  lungs  ;  into 
the  right  the  impure  blood  from  the  body.  To  some 
extent  these  mix  as  they  are  forced  into  the  general  cir- 
culation by  the  single  ventricle.  The  amount  of  oxygen 
carried  is  therefore  smaller  than  in  the  higher  air-breathers, 
the  amount  of  energy  is  proportionately  less,  and  hence  it 
is  that  all  are  cold-blooded  and  of  comparatively  sluggish 
habits. 

In  some  species  of  salamanders  the  lungs  may  also  dis- 
appear, and  breathing  is  carried  on  by  the  skin,  as  it  is  to 
a  certain  extent  in  all  amphibians.  In  the  frogs  and  toads 
lungs  are  invariably  present,  and  vocal  organs  are  situated 
at  the  opening  of  the  windpipe  in  the  throat.  These  pro- 
duce the  characteristic  croaking  and  shrilling,  which  in 
many  species  are  intensified  through  the  agency  of  one  or 
two  large  sacs  communicating  with  the  mouth-cavity. 

Although  the  brain  is  small  in  the  amphibians,  it  is 
more  complex  in  several  respects  than  it  is  in  fishes. 
The  eyes  are  also  usually  well  developed,  but  in  some  of 
the  cave  and  burrowing  salamanders  they  are  concealed 
beneath  the  skin,  and  are  rudimentary.  The  ear  varies 
considerably  in  complexity  in  the  different  species,  but  in 
the  possession  of  semicircular  canals  and  labyrinth  resem- 
bles that  of  the  fishes.  In  the  frogs  and  toads,  as  one  may 
readily  discover,  the  drum  or  tympanum  is  external,  ap- 


182  ANIMAL  FORMS 

pearing  as  a  smooth  circular  area  behind  the  eye.  Organs 
of  touch,  smell,  and  taste  are  likewise  developed  in  varying 
degree  of  perfection. 

174.  Breeding-habits. — While  the  great  majority  of  am- 
phibians mate  in  the  spring  and  deposit  their  eggs  in  the 
water,  often  to  the  accompaniments  of  croakings  and  pip- 
ings almost  deafening  in  intensity,  several  species,  for 
various  reasons,  have  adopted  different  methods.  Some  of 
the  salamanders  bring  forth  young  alive,  and  several  species 
of  toads  and  frogs  are  known  in  which  the  young  are  cared 
for  by  the  parent  until  their  metamorphosis  is  complete. 
In  one  of  the  European  toads  (Alytes)  the  male  winds 
the  strings  of  eggs  about  his  body  until  the  tadpoles  are 


FIG.  110.— Salamanders.  The 
axolotl  (the  larva  of  Am- 
blystoma  tigrinitrri)  and 
the  newt  (Diemyctylus  to- 
rosus).  P^K , 

ready  to  hatch  ;  and  in  a  few  species  of  tree-toads  the  eggs 
are  stored  in  a  great  pouch  on  the  back  of  the  parent  until 
the  early  stages  of  growth  are  over.  In  the  Surinam  toad 
of  South  America  the  eggs  are  placed  by  the  male  on  the 
back  of  the  female,  and  each  sinks  into  a  cavity  in  the 
spongy  skin.  Here  they  pass  through  the  tadpole  stage 
without  the  usual  attendant  dangers,  and  emerge  with  the 
form  of  the  adult. 


THE  AMPHIBIANS  183 

Sunlight  and  warmth  are  apparent  necessities  for  speedy 
development.  Tadpoles  kept  in  captivity  where  the  con- 
ditions are  generally  unfavorable  may  require  years  to  as- 
sume the  adult  form.  As  mentioned  above,  the  tiger  sala- 
mander (Amblystoma  tigrinum)  occurs  in  most  parts  of  the 
United  States  and  Mexico.  In  the  East  this  species  drops  its 
gills  in  early  life  as  other  salamanders  do,  and  assumes  the 
adult  form,  but  in  the  cold  water  of  high  mountain  lakes, 
in  Colorado  and  neighboring  States,  it  may  never  become 
adult,  always  remaining  as  in  Fig.  110.  This  peculiar  form 
is  locally  known  as  axolotl.  In  this  condition  it  breeds.  It 
is  thus  one  of  the  very  few  examples  of  animals  whose  un- 
developed larvae  are  able  to  produce  their  kind.  Owing  to 
this*  trait  it  was  at  first  considered  a  distinct  species,  and 
many  years  elapsed  before  its  relationship  to  the  true  adult 
form  was  discovered. 


CHAPTER  XVI 

THE    REPTILES 

175.  General  characteristics.— In  all  the  reptiles  the  gen- 
eral shape  of  the  body,  and  to  some  extent  the  internal 
plan,  is  not  materially  different  from  that  seen  among  the 
amphibians.  In  spite  of  external  resemblance  the  actual 
relationship  is  not  very  close.  It  appears  to  be  true  that 
ages  ago  the  ancestors  of  the  modern  reptiles  were  aquatic 
animals,  possibly  somewhat  similar  to  some  of  the  sala- 
manders; but  they  have  become  greatly  changed,  and 
are  now,  strictly  speaking,  land  animals.  At  no  time  in 
their  development  after  leaving  the  egg  do  we  find  them 
living  in  the  water  and  breathing  by  gills.  Some  species, 
such  as  the  turtles,  lead  aquatic  or  semiaquatic  lives,  but 
the  modifications  which  fit  them  for  such  an  existence 
render  them  only  slightly  different  from  their  land-inhabit- 
ing relatives.  The  skin  bears  overlapping  scales  or  horny 
plates,  united  edge  to  edge,  as  in  the  turtles,  enabling  them 
to  withstand  the  attacks  of  enemies  and  the  effects  of  heat 
and  dryness.  Indeed,  it  is  when  heat  is  greatest  that  rep- 
tiles are  most  active.  In  no  other  class  of  vertebrates,  and 
very  few  invertebrates,  do  normal  activities  of  the  body 
appear  to  be  so  directly  dependent  upon  external  warmth. 
In  the  presence  of  cold  they  rapidly  grow  sluggish,  and 
sink  into  a  dormant  state. 

As  in  the  case  of  all  animals,  habits  depend  upon 
structure,  and  accordingly  among  the  reptiles  we  find 
many  remarkable  modifications,  enabling  them  to  lead 
184 


THE   REPTILES 


185 


widely  different  lives.     Nevertheless  all   are   constructed 
upon  much  the  same  plan. 

176.  The  lizards  (Sauria).— As  in  the  amphibians,  es- 
pecially the  salamanders,  the  body  (Fig.  Ill)  consists  of 
a  relatively  small  head  united  by  a  neck  to  the  trunk, 


FIG.  111.— Common  lizard  or  swift  (Scelop&rus  undulatus).    Photograph  by  W.  II. 

FISHER. 

which,  in  turn,  passes  insensibly  into  a  tail,  usually  of  con- 
siderable length.  Two  pairs  of  limbs  are  almost  always 
present,  and  these  exhibit  the  same  skeletal  structure  as 
in  the  amphibians;  but  in  their  construction,  as  in  the 
other  divisions  of  the  body,  we  note  a  grace  of  propor- 
tion and  muscular  development  which  enable  the  lizards 
to  execute  their  movements  with  an  almost  lightning-like 
rapidity.  The  mouth  is  large  and  slit-like,  well  armed  with 
teeth,  and  the  eyes  and  ears  are  keen.  Scales  of  various 


186  ANIMAL  FORMS 

forms  and  sizes,  always  of  definite  arrangement,  cover  the 
body.  The  scales  are  always  colored,  in  some  species  as 
brilliantly  as  the  feathers  of  birds,  and  usually  harmonize 
with  the  surroundings  of  the  animal,  enabling  it  to  escape 
the  attacks  of  its  many  enemies.  Altogether  the  lizards 
are  a  very  attractive  group  of  animals.  As  in  the  salaman- 
ders, the  vertebral  column  is  usually  of  considerable  length, 
but  it  too  presents  a  lighter  appearance  and  a  greater  flexi- 
bility. Slender  ribs  are  present,  and  a  breast-bone  and  the 
girdles  which  support  the  limbs.  Although  more  ossified 
than  in  the  amphibians,  the  skull  still  continues  to  be  com- 
posed here  and  there  of  cartilage.  The  roof  also  is  yet 
incomplete,  but  with  the  firm  plates  on  the  surface  of  the 
head  ample  protection  is  afforded  the  small  brain  under- 
neath. As  above  mentioned,  the  limbs  are  slender  and 
insufficient  to  support  the  body,  which  accordingly  rests 
upon  the  ground,  and  by  its  wrigglings  and  the  pushing  of 
the  limbs  is  borne  from  place  to  place.  It  will  be  recalled 
that  some  of  the  salamanders  living  in  subterranean  haunts 
and  burrowing  in  the  soil  have  no  need  of  limbs,  and  the 
latter  have  accordingly  disappeared.  This  condition  is 
paralleled  by  certain  species  of  lizards.  The  blindworms 
(which  are  neither  blind  nor  worms,  but  true  lizards,  though 
snake-like  in  appearance)  are  devoid  of  limbs,  as  are  also 
the  "  glass-snakes."  In  some  species  the  hinder  pair  arise 
in  early  life,  but  they  remain  small,  and  ultimately  disap- 
pear. In  almost  all  lizards  the  tail  is  very  brittle,  breaking 
at  a  slight  touch.  In  such  case  the  lost  member  will  grow 
again  after  a  time. 

177.  The  snakes  (Serpentes).— The  snakes  are  character- 
ized by  a  cylindrical,  generally  greatly  elongated  body,  in 
which  the  divisions  into  head,  neck,  trunk,  and  tail  are  not 
sharply  defined.  As  we  have  seen,  this  is  also  true  of  cer- 
tain lizards,  but  the  naturalist  finds  no  difficulty  in  detecting 
the  differences  between  them.  Another  peculiarity  of  the 
snakes  is  in  the  great  freedom  of  movement  of  the  bones 


THE  REPTILES 


187 


not  concerned  with  the  protection  of  the  brain.  In  the 
reptiles  the  lower  jaw  does  not  unite  directly  with  the 
skull,  as  in  the  higher  animals,  but  to  an  intermediate 
bone,  the  quadrate,  which  is  attached  to  the  skull.  In  the 
snakes  these  unions  are  made  by  means  of  elastic  liga- 
ments. The  two  halves  of  the  lower  jaw  are  also  held 


FIG.  112. — Blacksnake  (Bascanion  constrictor).    Photograph  by  W.  H.  FISHER. 

together  by  a  similar  band,  so  that  the  entire  palate  and 
lower  jaw  are  loosely  hung  together.  This  enables  the 
snake  to  distend  its  mouth  and  throat  to  an  extraordinary 
degree,  and  to  swallow  frogs  and  toads  but  slightly  smaller 
than  itself.  Where  the  prey  is  of  relatively  small  size,  the 
halves  of  the  lower  jaw  alternate  with  each  other  in  pulling 
backward,  thus  drawing  the  food  down  the  throat.  The 
food  is  never  masticated.  The  teeth  are  usually  small  and 
recurved,  and  serve  only  to  hold  the  food  until  it  may  be 
swallowed.  The  latter  process  is  facilitated  by  the  copious 
secretion  of  the  salivary  glands,  which  become  very  active 
at  this  time. 

A  further  character  of  the  snakes  is  the  absence  exter- 


188  ANIMAL  FORMS 

nally  of  any  trace  of  limbs.  However,  in  some  of  the 
pythons  and  boas  hind  limbs  are  present  in  the  form  of 
small  groups  of  bones  embedded  beneath  the  skin  and  ter- 
minating in  a  claw.  There  thus  appears  to  be  no  doubt 
that  the  ancestors  of  the  modern  snakes  were  four-footed, 
lizard-like  creatures,  which  have  assumed  the  present  form 
in  response  to  the  necessity  of  adaptation  to  new  conditions. 

More  than  any  other  order  of  vertebrates  do  the  snakes 
deserve  the  name  of  creeping  things,  and  yet  their  method 
of  locomotion  enables  them  to  crawl  and  swim  with  a  ra- 
pidity equal  to  that  of  many  of  the  more  highly  developed 
animals.  This  depends  chiefly  upon  certain  peculiarities 
of  the  skeleton,  which  consists  merely  of  a  skull,  vertebral 
column,  and  ribs.  The  vertebrae,  usually  two  or  three  hun- 
dred in  number,  are  united  together  by  ball-and-socket 
joints,  and  each  attaches  by  similar  joints  a  pair  of  slender 
ribs.  These  in  turn  are  attached  to  the  broad  outer  plates 
upon  which  the  body  rests,  and  the  whole  system  is  operated 
by  a  powerful  set  of  muscles.  Upon  the  contraction  of  the 
muscles  the  ventral  plates  are  made  to  strike  backward 
upon  the  ground  or  other  rough  surface,  which  drives  the 
body  forward.  Also,  the  ribs  may  be  made  to  move  back- 
ward and  forward,  and  the  snake  thus  progresses  like  a 
centiped  or  "  thousand-legs." 

178.  The  turtles  (Chelonia). — In  many  respects  the  tur- 
tles are  the  most  highly  modified  of  all  the  reptiles.  The 
body  (Fig.  113)  is  short  and  wide  and  enclosed  in  a  shell  or 
heavy  armor,  consisting  of  an  upper  portion,  the  carapace, 
and  a  flat  ventral  plate,  the  plastron.  The  shape  of  the 
carapace  varies  greatly  from  a  low,  flat  shield  to  a  highly 
vaulted  dome,  remaining  cartilaginous  throughout  life,  as 
in  the  soft-shelled  turtles,  or  becoming  bony  and  of  great 
strength.  The  two  portions  of  the  shell  form  a  box-like 
armor  through  whose  openings  may  be  extended  the  head, 
tail,  and  limbs.  As  a  means  of  protection  the  turtle  may 
retract  these  organs  within  the  shell.  The  head  is  generally 


THE  REPTILES 


189 


thick-set  and  muscular,  and  provided  with  horny  jaws 
entirely  destitute  of  teeth,  like  those  of  the  birds.  The 
limbs  also  are  usually  short  and  thick  and  variously  shaped, 
and  adapted  for  aquatic  or  terrestrial  locomotion.  The 
number  of  vertebrae  in  the  body  and  tail  are  relatively  few, 
and  the  thick  and  heavy  body  is  devoid  of  the  elements  of 
grace  and  agility  of  movement  characteristic  of  the  other 
reptiles.  On  the  other  hand,  the  former  enjoy  a  freedom 
from  the  attacks  of  enemies  not  accorded  to  animals  in 
general. 

At  first  sight  the  appearance  of  a  turtle  does  not  indi- 
cate a  close  relationship  to  the  other  reptiles,  but  a  more 


FIG.  113. — Box -turtle  (Terraptne  carollr 


careful  examination,  and  especially  of  their  development, 
discloses  a  remarkable  resemblance.  The  head,  tail,  and 
limbs  are  essentially  similar  to  those  of  the  lizards,  but  in 
the  trunk  region  peculiar  modifications  have  taken  place. 
The  ribs  at  first  separate,  as  in  other  animals,  flatten 
greatly,  and  unite  with  a  number  of  bones  embedded  in 
the  skin,  thus  forming  one  great  plate  overlying  the  back 
of  the  animal.  About  the  circumference  of  the  shield 
other  dermal  or  skin-bones  are  added,  which  increase  the 
area  of  the  carapace,  and  at  the  same  time  still  others  have 


190  ANIMAL  FORMS 

arisen  and  united  on  the  ventral  surface  to  form  the  plas- 
tron. In  this  process  the  shoulder-  and  hip-girdles  which 
attach  the  limbs  come  to  be  withdrawn  into  the  body,  and 
we  have  the  curious  example  of  an  animal  enclosed  within 
its  back-bone  and  ribs.  This  is  even  more  the  case  with 
the  box-turtles  (Fig.  113),  common  in  the  eastern  United 
States,  whose  ventral  plate  is  hinged  so  that  after  the 
limbs,  head,  and  tail  have  been  withdrawn  it  may  be  made 
to  act  like  a  lid  to  completely  enclose  the  fleshy  parts  of 
the  body. 

Scales  and  horny  plates  are  present,  as  in  other  reptiles, 
the  former  covering  all  parts  of  the  body  except  the  cara- 
pace and  plastron,  which  support  the  plates.  In  nearly  all 
species  the  latter  are  of  considerable  size,  and  in  the  tor- 
toise-shell turtles  are  valuable  articles  of  commerce.  They 
also  are  sculptured  in  a  fashion  characteristic  of  each  spe- 
cies, and  may,  like  the  colors  of  other  animals,  render  them 
more  like  their  surroundings,  and  consequently  incon- 
spicuous. 

179.  Crocodiles  and  alligators  (Crocodilia). — The  alligators 
(Fig.  114)  and  crocodiles  are  much  more  complex  in  struc- 
ture than  the  lizards,  though  their  general  form  is  much  the 
same.  The  body  is  covered  with  an  armor  of  thick  bony 
shields  and  horny  scales.  These,  along  the  median  line,  are 
keeled,  and  extending  along  the  length  of  the  laterally  com- 
pressed tail  form  an  efficient  swimming  organ  and  rudder. 
The  mouth  is  of  large  size,  and  is  bounteously  supplied  with 
large  conical  teeth,  which  are  set  in  sockets  in  the  jaw,  and 
not  fused  with  it,  as  in  many  of  the  lizards.  The  nose  and 
ears  may  be  closed  by  valves  to  prevent  the  entrance  of 
water,  and  a  similar  structure  blocks  its  passage  beyond 
the  throat  while  the  mouth  is  open.  When  large  animals, 
such  as  hogs  or  calves,  are  captured  as  they  come  to  drink, 
these  devices  enable  the  alligator  or  crocodile  to  sink  with 
them  to  the  bottom  and  hold  them  until  drowned.  The 
limbs,  short  and  powerful,  are  efficient  organs  of  locomo- 


THE  REPTILES  191 

tion  on  land,  and  together  with  the  general  shape  of  the 
body,  are  also  well  adapted  for  swimming. 


FIG.  114.— Alligator  (Alligator  mississippiensis). 

180.  Distribution  of  the  lizards.— In  a  general  way  the 
number  of  reptiles  is  greatest  where  the  temperature  is 
highest.  The  tropics  therefore  abound  in  species,  often 
of  large  size,  and  usually  of  bright  coloration.  As  one 
travels  northward  the  numbers  rapidly  diminish,  their  size 
is  smaller,  and  the  tints  less  pronounced.  In  all  probability 
not  less  than  four  thousand  known  reptiles  exist,  whose 
haunts  are  of  the  most  varied  description. 

In  North  America  the  lizards  are  almost  exclusively 
confined  to  the  southern  portions,  only  a  very  few  species 
extending  up  to  the  fortieth  parallel.  Among  these  the 
skinks  (Eumeces)  are  most  widely  distributed.  The  blue- 
tailed  skink  is  probably  the  most  familiar,  a  small  lizard 
eight  or  ten  inches  in  length,  dark  green  with  yellowish 
streaks  and  a  bright-blue  tail.  On  sunny  days  it  may 
sometimes  be  seen  darting  about  on  the  bark  of  trees  in 
search  of  insects,  upon  which  it  feeds. 

On «  of  the  most  familiar  lizards  in  this  country  is  the 
"  glass-snake,v  found  burrowing  in  the  drier  soil  of  the 
southern  half  of  the  United  States  east  of  the  Mississippi. 
35 


192  ANIMAL  FORMS 

Both  pairs  of  limbs  are  absent,  but  by  wriggling  movements 
of  the  body  this  lizard  is  able  to  force  its  way  through  light 
soil  with  considerable  rapidity.  It  is  a  matter  of  some 
difficulty  to  secure  entire  specimens,  for  with  other  than 
the  gentlest  handling  the  tail  severs  its  connection  with 
the  body,  as  the  vertebrae  in  this  portion  are  extremely 
brittle.  This  peculiarity,  together  with  its  shape,  has  given 
it  the  popular  name  of  glass-snake.  Many  species  of  liz- 
ards will  thus  detach  the  tail,  a  habit  which  is  a  means  of 
protection,  enabling  the  animal  to  scamper  away  into  a 
place  of  safety  while  its  enemy  is  concerning  itself  with 
the  detached  member.  Later  on  a  new  tail  develops, 
though  usually  of  a  less  symmetrical  form. 

181.  Horned  toads. — The  horned  toads  (Phrynosoma)  are 
lizards  peculiar  to  the   hot,  sandy  deserts  and  plains  of 


FIG.  115.— Gila  monster  (Heloderma  suspectum).    One-third  natural  size. 

Mexico  and  the  western  United  States.  The  body  is  com- 
paratively broad  and  flat,  almost  toad-like,  and  is  covered 
with  scales  and  spines  of  brownish  and  dusky  tint,  so  Iik3 
dried  sticks  and  cactus  spines  in  form  and  color  as  to  ren- 
der them  difficult  of  detection.  In  captivity  they  readily 


UNIVERSITY 

OF 


THE  REPTILES  193 

adapt  themselves  to  their  new  surroundings,  become  tame, 
and  feast  on  flies,  ants,  and  other  insects,  which  they  cap- 
ture by  the  aid  of  their  long  tongue.  The  horned  toads 
are  perfectly  harmless  creatures,  but  when  irritated  some- 
times perform  the  remarkable  feat  of  spurting  a  stream  of 
blood  from  the  eye  toward  the  intruding  object  for  a  dis- 
tance of  several  inches.  This  has  been  regarded  by  some 
as  a  zoological  fable ;  but  there  are  many  who  have  watched 
the  horned  toad  in  its  natural  state  and  in  captivity,  and 
they  assure  us  that  it  is  a  fact. 

In  the  hot  deserts  of  Arizona  and  Sonora  is  another 
peculiar  species  of  lizard  known  as  the  Gila  monster  (Hclo- 
derma)  (Fig.  115),  having  the  distinction  of  being  the  only 
poisonous  lizard  known.  Further  protection  is  afforded 
by  bony  tubercles  on  the  head  and  by  scales  over  the 
remainder  of  the  body,  all  of  which  are  colored  brown  or 
various  shades  of  yellow,  giving  the  animal  a  peculiar 
streaked  and  blotched  appearance. 

182.  Distribution  of  the  snakes. — The  snakes  are  much 
more  common  than  the  lizards.  All  over  the  United  States 
one  meets  with  them,  especially  the  garter-  or  water-snakes. 
Of  less  wide  distribution  are  the  black-,  grass-,  and  milk- 
snakes,  and  a  number  of  less  known  species,  all  of  which 
are  perfectly  harmless  and  often  make  interesting  pets. 
Some  of  them  when  cornered  show  considerable  temper, 
flatten  the  head  and  hiss  violently,  and  imitate  poisonous 
forms,  but  venomous  snakes  are  comparatively  few  in  num- 
ber in  northern  and  eastern  United  States.  In  the  south- 
ern portions  of  the  country  they  become  more  abundant. 
Along  the  streams  and  in  the  swamps  the  copperheads,  and 
especially  the  water-moccasins,  often  lie  in  wait  for  frogs 
and  fish.  Both  these  species  are  especially  dreaded,  as  they 
strike  without  giving  any  warning  sound,  but  the  name 
and  bad  reputation  of  the  moccasin  is  often,  especially  in 
the  South,  transferred  to  perfectly  harmless  water-snakes. 
On  higher  ground  are  the  rattlesnakes  (Crotalus),  once 


194 


ANIMAL  FORMS 


abundant  but  now  in  many  regions  well-nigh  exterminated. 
In  these  species  the  tail  terminates  in  a  series  of  horny 


FIG.   116.— Diamond-rattlesnake   (Crotalus   adamanteus\     Photograph   by  W.   H. 

FISHER. 

rings  that  produce  a  buzzing  sound  like  that  of  the  locust 
when  the  tail  is  rapidly  vibrated. 

183.  Distribution  of  the  turtles, — The  turtles  are  perhaps 
somewhat  less  dependent  upon  warmth  than  other  reptiles, 
yet  they  too  delight  to  bask  in  the  sunshine,  and  soon  grow 
sluggish  in  its  absence.  In  all  our  fresh-water  streams  and 
ponds  they  are  familiar  objects,  and  several  species  extend 
up  into  Canada.  Among  the  turtles  the  soft  shell,  the 
painted  and  the  snapping  turtles  have  the  widest  distri- 
bution, scarcely  a  good-sized  stream  or  pond  from  the  Gulf 
of  Mexico  to  Canada,  and  even  farther  north,  being  without 
one  or  more  representatives.  All  are  carnivorous  and  vora- 
cious, and  the  snapping  turtles  are  especially  ferocious,  and 
"  for  their  size  are  the  strongest  of  reptiles."  In  the  woods 
and  meadows  the  wood-tortoise  and  box-turtles  are  occa- 


THE  REPTILES 


195 


sionally  met  with,  and  at  sea  several  turtles  exist,  some  of 
them  of  great  size.  Among  these  is  the  leather-turtle, 
found  in  the  warmer  waters  of  the  Atlantic,  lazily  floating 
at  the  surface  or  actively  engaged  in  capturing  food.  They 
attain  a  length  of  from  six  to  eight  feet,  and  a  weight  of 
over  a  thousand  pounds,  and  are  sometimes  captured  for 
food  when  they  come  ashore  to  bury  their  eggs  in  the  sand. 
By  this  same  method  the  loggerheads,  the  hawkbills,  and 
the  common  green  turtles  are  also  captured  in  consider- 
able numbers.  These  are  of  smaller  size,  and  the  second 
named  is  of  considerable  value,  as  the  horny  plates  cover- 


FIG.  117.— Hawkbill  turtle  (Eretmochdys  imbricata). 

ing  the  shell  furnish  the  tortoise-shell  of  commerce.  These 
plates  are  removed  after  the  animal  is  killed,  by  soaking 
in  warm  water  or  by  the  application  of  heat. 

184.  Food  and  digestive  system. — Some  reptiles,  among 
which  are  a  number  of  species  of  lizards  and  the  box-  and 
green  turtles,  are  vegetarians,  but  the  great  majority  are 


196  ANIMAL  FORMS 

carnivorous,  and  usually  very  voracious.  The  lizards  espe- 
cially devour  large  quantities  of  insects  and  snails,  together 
with  small  fishes  and  frogs.  The  latter  figure  largely  in 
the  turtle's  bill  of  fare,  and  in  that  of  the  snakes,  which 
also  capture  birds  and  mammals.  On  the  other  hand,  many 
of  the  reptiles  prey  upon  one  another  ;  and  they  are  the 
favorite  food  of  hawks  and  owls  and  numerous  water-birds, 
of  skunks  and  weasels  and  many  other  animals,  which  look 
for  them  continually.  Many  of  the  turtleSj  owing  to  their 
protective  armor,  and  the  snakes  because  of  their  poison- 
ous bite  or  great  size  and  strength,  are  more  or  less  ex- 
empt, but  this  is  not  true  of  their  eggs  and  young.  The 
smaller  species  depend  upon  keenness  of  sense,  agility,  and 
inconspicuous  tints.  These  latter  may  undergo  changes 
according  to  the  character  of  the  surroundings,  but  usually 
only  to  a  slight  extent.  The  chameleons  of  the  tropics 
and  a  similarly  colored  green  lizard  on  the  pine-trees  in 
the  Southern  States  are  able  to  change  with  great  rapidity 
from  green,  through  various  shades,  to  brown. 

185.  Respiration  and  circulation.  —  While  still  in  the  egg 
the  young  lizard  develops  rudimentary  gills,  and  thus  bears 


extna. 
int 


FIG.  118.— Dissection  of  lizard  (Sceloporus).  an.,  anal  opening  ;  au.,  auricle  ;  crb.h., 
brain  ;  coec.,  intestine  ;  kd.,  kidney  ;  Ling.,  left  lung  ;  lr.,  liver ;  pn.,  pancreas  ; 
sp.c.,  spinal  cord  ;  spl.,  spleen  ;  st.,  stomach ;  v.,  ventricle  of  heart. 

evidence  to  the  fact  that  its  distant  ancestors  were  aquatic ; 
but  before  hatching  they  disappear,  and  lungs  arise,  which 


THE  REPTILES  197 

remain  functional  throughout  life.  Corresponding  to  the 
shape  of  the  body,  these  are  usually  much  elongated  and 
ordinarily  paired  (Fig.  118,  l.lng.).  The  snakes  are  peculiar 
in  having  the  left  lung  rudimentary  or  even  lacking  com- 
pletely, while  the  right  one  becomes  greatly  elongated  and 
extends  far  back  into  the  body.  In  nearly  all  the  reptiles 
the  amount  of  oxygen  brought  into  the  lungs  is  relatively 
large  and  the  activity  of  the  animal  is  proportionately 
great.  The  circulation  of  reptiles  shows  an  advance  be- 
yond that  of  the  Amphibia.  As  in  the  latter,  there  are 
two  distinct  auricles ;  but  the  chief  difference  arises  from 
the  fact  that  the  ventricle  is  more  or  less  divided  by  a  par- 
tition which  to  a  considerable  degree  prevents  the  blood 
returning  from  the  lungs  from  mixing  with  the  impure 
blood  as  it  returns  from  its  journey  over  the  body.  In  the 
crocodiles  and  alligators  the  partition  is  complete,  and  the 
circulation  thus  approaches  close  to  that  of  the  higher 
animals. 

186.  Hibernation. — Attention  has  already  been  called  to 
the  fact  that  reptiles  are  very  susceptible  to  cold,  rapidly 
growing  less  active  as  the  temperature  lowers.     When  win- 
ter comes  on  they  seek  protected  spots,  and  either  alone 
or  grouped  together  hibernate.     The  various  activities  of 
the  body  during  this  period  are  at  very  low  ebb.     The  blood 
barely    circulates,   breathing    is  imperceptible,   and   stiff 
and  insensible  to  the  world  about  them  they  remain  until 
the   warmth   again   stirs    them   to  their  former   activity. 
Some  of  our  common  turtles  must  also  pass  a   somewhat 
similar  sleep  while  embedded  far  down  in  the  mud  during 
the  disappearance  of  the  ponds  in  summer.     At  such  times 
no  food  is  taken,  but  owing  to  their  loss  in  weight  it  is 
probable  that  a  slow  consumption  of  the  body  supplies  the 
small  amount  of  necessary  energy. 

187.  Nervous  system  and  sense-organs.— At  first  sight  one 
is  struck  with  the  small  size  of  the  brain  of  fishes,  Am- 
phibia, and  reptiles.     Their  intelligence  likewise  is  at  low 


198  ANIMAL  FORMS 

ebb.  Almost  all  the  movements  and  operations  of  the  body 
appear  to  be  carried  on  by  the  animal  with  little  apparent 
thought.  Their  acts,  like  most  of  the  animals  below  them, 
are  said  to  be  instinctive ;  'yet  they  are  sufficiently  well  done 
to  enable  the  animal  to  procure  its  food,  avoid  its  enemies, 
and  lead  a  successful  life.  As  is  true  of  other  animals,  the 
ability  of  the  reptile  to  cope  with  its  surroundings  depends 
to  a  great  extent  upon  the  keenness  of  one  or  all  of  its  or- 
gans of  special  sense.  In  the  reptiles  the  sense  of  sight  is 
perhaps  sharpest,  but  there  is  considerable  variation  in  this 
respect.  Movable  eyelids  are  present  in  most  lizards,  to- 
gether with  a  third,  known  as  the  nictitating  membrane,  a 
thin,  transparent  fold  located  at  the  inner  angle  of  the  eye, 
over  which  it  is  drawn  with  great  rapidity.  Snakes  have 
no  movable  eyelids,  hence  the  eye  has  a  peculiar  stare. 
Furthermore,  their  sense  of  sight,  except  in  a  few  tree-dwell- 
ing species,  appears  to  be  defective,  the  majority  depending 
largely  upon  the  sense  of  touch. 

In  all  the  vertebrates  a  very  peculiar  organ  known 
as  the  pineal  gland  or  eye  is  situated  on  the  roof  of  the 
brain.  In  several  lizards  its  position  is  indicated  by  a  trans- 
parent area  in  one  of  the  plates  of  the  head,  and  by  an 
opening  in  the  bones  of  the  roof  of  the  skull.  In  young 
reptiles,  and  especially  in  one  of  the  New  Zealand  lizards 
(Hatt&ria,  Fig.  119),  its  resemblance  to  an  eye  is  decidedly 
striking.  Lens,  retina,  pigment,  cornea,  are  all  present 
much  as  they  are  in  some  of  the  snails,  but  they  finally 
degenerate  more  or  less  as  the  animal  reaches  maturity. 
It  is  a  general  belief  that  it  represents  the  remnant  of  an 
organ  of  sight,  a  third  eye,  which  looked  out  through  the 
roof  of  the  skull  in  some  of  the  ancient  vertebrates. 

With  the  possible  exception  of  the  few  species  of  reptiles 
which  produce  sounds,  probably  to  attract  their  mate,  the 
sense  of  hearing  is  not  particularly  well  developed.  The 
senses  of  smell  and  taste  are  also  comparatively  feeble.  The 
latter  sense  is  located  in  the  tongue,  which  is  also  popularly 


THE   REPTILES  199 

supposed  to  serve  for  the  purpose  of  defense,  and  that  it  is 
in  some  way  related  to  the  poison-glands.  This,  however, 
is  an  error.  The  tongue  is  used  primarily  as  an  organ  of 


FIG.  119.—  Tuatera  (Sphenodan  punctatus). 

touch,  and  in  snakes  especially  it  is  almost  continually 
darted  in  and  out  to  determine  the  character  of  the  animal's 
surroundings. 

188.  Egg-laying.— The  eggs  of  the  reptiles  are  relatively 
large  and  enclosed  in  a  shell  like  a  bird's  egg,  the  shell, 
however,  being  leathery  rather  than  made  of  lime.  These 
are  deposited  in  some  warm  situation,  and  generally  left  to 
themselves  to  hatch.  Under  stones,  logs,  and  leaves,  or 
buried  lightly  in  the  soil,  are  the  positions  most  frequently 
chosen  by  the  lizards  and  snakes.  The  turtles  almost 
invariably  select  the  warm  sand  at  the  edge  of  the  water, 
and  after  scooping  a  hole  lay  numerous  spherical  eggs, 
usually  at  night.  The  alligators  lay  upward  of  a  hundred 
eggs  about  the  size  of  those  of  a  goose,  and  guard  them 
jealously  until  and  even  after  they  hatch.  On  the  other 
hand,  the  young  of  many  lizards  and  snakes  are  born  alive, 
the  eggs  being  hatched  within  the  body. 

Many  reptiles  are  surprisingly  slow  in  attaining  maturity, 
and  live  to  an  age  attained  by  few  other  animals.  It  is  a 
well-known  fact  that  turtles  live  fully  a  hundred  years,  and 


200  ANIMAL  FORMS 

probably  the  same  is  true  of  the  crocodiles  and  alligators 
and  some  of  the  larger  snakes.  Their  enemies  are  few,  and 
death  usually  results  when  the  natural  course  is  run. 

Throughout  life  all  reptiles  periodically  shed  their  skin, 
as  birds  do  their  feathers  and  mammals  their  fur.  In  the 
snakes  and  some  of  the.lizards  the  skin  at  the  lips  loosens, 
and  the  animal  gradually  slips  out  of  its  old  slough,  bright 
and  glossy  in  the  new  one  which  previously  developed.  In 
the  others  the  old  skin  hangs  on  in  tatters,  gradually  com- 
ing away  as  they  scamper  through  the  grass. 


CHAPTEE  XVII 

THE    BIRDS 

189.  Characteristics. — Birds  form  one  of  the  most  sharp* 
ly  defined  classes  in  the  animal  kingdom,  and  the  variations 
among  the  different  species  are  relatively  small.  "The 
ostrich  or  emu  and  the  raven,  for  example,  which  may  he 
said  to  stand  at  opposite  ends  of  the  series,  present  no  such 
anatomical  differences  as  may  be  found  between  a  common 
lizard  and  a  chameleon,  or  between  a  turtle  and  a  tortoise," 
and  these  we  know  to  be  relatively  slight. 

In  many  respects  the  birds  resemble  the  reptiles,  and 
long  ago  in  the  world's  history  the  relationship  was  much 
closer  than  now,  as  we  know  from  certain  fossil  remains  in 
this  country  and  in  Europe.  One  of  the  earliest  of  these 
fossil  birds,  the  Archaeopteryx,  is  a  most  remarkable  com- 
bination of  bird  and  lizard.  Unlike  any  modern  bird,  the 
jaws  were  provided  with  many  conical  reptile-like  teeth. 
The  wings  were  rather  small,  and  the  fingers,  tipped  with 
claws,  were  distinct,  not  grown  together,  as  in  modern  birds. 
The  tail  was  as  long  as  the  body,  and  many-jointed,  like  a 
lizard's,  each  vertebra  carrying  two  long  feathers.  The 
bird  was  about  the  size  of  a  crow,  and  it  probably  could 
not  fly  far.  Other  ancient  types  have  been  discovered — 
principally  sea-birds — many  of  which  existed  when  the 
Pacific  extended  over  the  region  now  occupied  by  the 
Rocky  Mountains.  These  were  all  of  the  same  generalized 
type,  intermediate  between  reptile  and  bird.  This  fact 
leads  us  to  the  belief  that  birds  descended  from  reptilian 

201 


202  ANIMAL  FORMS 

ancestors,  and  in  becoming  more  perfectly  adapted  for  an 
aerial  life  have  developed  into  our  modern  forms. 

In  the  modern  birds  the  most  important  peculiarities, 
those  which  separate  them  from  all  other  animals,  are 
correlated  with  the  power  of  flight.  The  body  is  spindle- 
shaped,  for  readily  cleaving  the  air.  The  fore  limbs  serve 
as  wings.  The  hind  limbs,  supporting  the  weight  of  the 
body  from  the  ground,  are  usually  well  developed.  A  series 
of  air-chambers  usually  exists  in  powerful  fliers.  This 
serves  a  purpose  analogous  to  that  of  the  air-bladder  of  a 
fish,  giving  buoyancy.  But  the  most  characteristic  mark 
of  a  bird,  as  above  stated,  is  its  feathers,  universally  present 
and  never  found  outside  the  class.  Like  the  scales  of 
lizards,  and  probably  derived  from  similar  structures,  the)' 
are  of  different  forms,  and  serve  a  variety  of  purposes. 
The  larger  ones,  with  powerful  shafts,  and  forming  the  tail, 
act  as  a  rudder.  Those  of  the  wings  give  great  expanse 
with  but  little  increase  in  weight,  and  are  so  constructed 
that  upon  the  down-stroke  they  offer  great  resistance  to 
the  air,  and  push  the  bird  forward,  while  in  the  reverse 
direction  the  air  slips  through  them  readily.  In  flight 
these  movements  of  the  wing  may  be  too  rapid  for  us  to 
follow,  as  in  the  humming-birds,  though  they  are  usually 
much  slower,  two  to  five  hundred  a  minute  in  many  power- 
ful fliers,  such  as  the  ducks,  and  frequently  long-continued 
enough  to  carry  them  many  hundreds  of  miles  at  a  single 
flight.  The  remaining  feathers  are  soft  and  downy,  giving 
roundness  to  the  body  and  enabling  it  to  cleave  the  air  with 
greater  ease,  and,  being  poor  conductors  of  heat,  they  aid  in 
keeping  the  body  at  the  high  temperature  characteristic  of 
birds.  In  most  birds  the  body  is  not  uniformly  clothed  in 
feathers.  Naked  spaces,  usually  hidden,  intervene  between 
the  feather  tracts,  and  on  the  feet  and  toes  scales  exist. 

190.  Molting. — As  we  all  know,  the  growth  of  feathers, 
unlike  that  of  hair  and  nails,  is  limited,  and  after  they  have 
become  faded  and  worn  out  they  are  shed,  and  new  ones 


THE  BIRDS  203 

arise  to  take  their  place.  This  process  of  molting  is 
usually  accomplished  gradually,  without  diminishing  the 
powers  of  flight ;  but  in  the  ducks  and  some  other  birds  all 
the  wing-  and  tail-f oathers  drop  out  simultaneously,  leaving 
the  bird  to  escape  its  enemies  by  swimming  and  diving. 
The  molting-process  usually  takes  place  in  the  fall,  after 
the  nesting  and  care  for  the  young  is  over,  and  often  when 
the  need  for  a  heavy  winter  coat  commences  to  be  felt. 
Many  birds  also  don  what  are  called  courting  colors,  ruffs, 
crests,  and  highly  colored  patches,  in  the  spring,  previous 
to  the  mating  season,  doubtless  for  the  purpose  of  attract- 
ing or  impressing  their  mates.  In  other  cases  the  change 
appears  to  be  related  to  the  bird's  surroundings.  A  most 
beautiful  example  of  this  is  the  ptarmigans — grouse-like 
birds  living  far  to  the  north.  During  winter  they  are  per- 
fectly white  and  are  almost  invisible  against  the  snow ;  but 
in  the  spring,  as  the  snow  disappears,  the  white  feathers 
gradually  fall  out  and  new  ones  arise.  The  latter  so  har- 
monize "with  the  lichen-colored  stones  among  which  it 
delights  to  sit,  that  a  person  may  walk  through  a  flock  of 
them  without  seeing  a  single  bird." 

There  are  also  numerous  birds,  chiefly  those  that  go  in 
flocks,  which  possess  what  are  known  as  color-calls  or  recog- 
nition-marks. These  may  consist  of  various  conspicuous 
spots  or  blotches  on  different  parts  of  the  head  or  trunk, 
such  as  we  see  in  the  yellowhammer  or  meadow-lark ;  or 
one  or  more  feathers  of  the  wings  or  tail  may  be  strikingly 
colored,  as  in  many  sparrows  and  warblers.  During  the 
time  the  bird  remains  at  rest  these  usually  are  concealed 
under  neighboring  feathers,  but  during  flight  they  are 
strikingly  displayed.  It  may  possibly  be  true,  as  many 
have  urged,  that  these  color-signals  are  for  the  purpose 
of  enabling  various  members  of  the  flock  to  readily  follow 
their  leader ;  but  this  and  many  other  interesting  questions 
regarding  the  color  of  birds  and  other  animals  have  not  yet 
received  final  answers. 


204  ANIMAL  FORMS 

In  very  many  animals,  fishes  as  well  as  birds,  the  tints 
on  the  under  side  of  the  body  are  usually  relatively  light 
colored,  shading  gradually  into  a  darker  tint  above.  This 
is  in  all  probability  a  protective  device,  as  was  recently 
shown  by  Mr.  A.  H.  Thayer,  an  American  artist.  His  ex- 
periments show  that  the  light  from  above  renders  the  back 
less  dark,  and  that  the  shadow  beneath  is  neutralized  by 
the  light  color.  The  bird  thus  appears  uniformly  lighted, 
and  this  effect,  together  with  streaks  and  blotches,  renders 
them  invisible  at  surprisingly  short  distances. 

191.  Skeleton. — Turning  now  to  the  internal  organization 
of  birds,  we  find  many  points  in  common  with  other  verte- 
brates, especially  the  reptiles,  but  many  interesting  modifi- 
cations are  also  present  that  adapt  them  for  flying  and  for 
collecting  their  food.  According  to  the  nature  of  the  food, 
the  beak  may  have  a  great  variety  of  forms.  The  skull  may 
be  thick  and  heavy,  or  thin  and  fragile,  but  these  are  mat- 
ters of  proportion  of  the  various  parts  possessed  by  all 
birds.  The  neck  also  is  of  differing  length ;  but  it  is  in  the 
trunk  region  that  the  greatest  changes  have  arisen,  as  we 
may  see  in  any  of  our  ordinary  birds.  For  example,  the 
vertebrae  of  this  part  of  the  body  are  more  or  less  fused 
together  into  rigid  framework,  to  which  are  attached  the 
ribs  that  in  turn  unite  with  the  breast-bone.  In  the  fliers 
the  latter  bears  a  vertical  plate  or  keel,  to  which  the  great 
muscles  that  move  the  wings  are  attached.  The  tail  con- 
sists, like  that  of  the  old-fashioned  birds,  of  several  verte- 
brae, but  these  are  of  small  size  and  fused  together  into  a 
little  knob  that  supports  the  tail-feathers.  The  fore  limbs 
are  used  for  flight,  but  there  are  the  same  bones  that  exist 
in  the  fore  limbs  of  other  vertebrates — one  for  the  upper 
arm,  two  for  the  lower,  a  thumb  carrying  a  few  feathers, 
and  known  as  the  bastard  wing,  and  indications  of  several 
bones  that  form  the  hand.  In  the  hind  limb  the  resem- 
blance is  equally  apparent,  though  its  different  parts  are 
of  relatively  large  size  to  support  the  body.  It  is  interest- 


THE  BIRDS  205 

ing  to  note  that  the  knee  has  been  drawn  far  up  into  the 
body,  and  that  the  joint  above  the  foot  is  in  reality  the 
ankle. 

We  thus  see  that  the  bird's  skeleton  presents  the  same 
general  plan  as  that  of  the  lizard,  for  example  ;  but  in  order 
to  combine  the  elements  of  strength,  lightness,  and  com- 
pactness essential  to  successful  flight,  it  has  been  necessary 
to  remodel  it  to  a  considerable  degree. 

192.  Other  internal  structures,— The  lungs  of  birds  con- 
sist of  two  dark-red  organs  buried  in  the  spaces  between  the 
ribs  along  the  back.  Each  communicates  with  extensive 
thin-walled  air-sacs  extending  into  the  space  between  the 


FIG.  120.— Anatomy  of  a  bird.  a«.,  auricle  ;,cbl.  and  crb.h.,  cerebellum  and  cerebral 
hemispheres  (divisions  of  the  brain)  ;  duo.,  intestine  (with  portion  removed) ; 
giz.,  gizzard  ;  kd.,  kidney  ;  r.lng ,  lung;  tr.,  trachea  or  windpipe  ;  vent.,  ven- 
tricle. 

various  organs,  and  in  many  birds  of  flight  they  even  extend 
into  the  bones  of  the  body,  and  thus  decrease  their  weight. 
"  The  enormous  importance  of  this  feature  to  creatures 
destined  to  inhabit  the  air  will  be  readily  understood  when 
we  learn  that  a  bird  with  a  specific  gravity  of  1.30  may 
have  this  reduced  to  only  1.05  by  pumping  itself  full  of  air." 
As  we  know,  air  is  taken  into  the  body  in  order  that  the 
oxygen  it  contains  may  combine  with  the  tissues  of  the 
body  to  liberate  the  energy  necessary  for  the  work  of  its 


206  ANIMAL  FORMS 

life.  The  life  of  birds  is  at  high  pressure,  hence  their  need 
of  much  oxygen.  They  habitually  breathe  deeper  breaths 
than  other  animals.  The  air  passing  into  the  body  trav- 
erses the  entire  extent  of  the  lung  on  its  way  back  to  the 
air-sacs,  with  the  result  that  large  quantities  of  oxygen  are 
taken  into  the  body.  This  is  distributed  by  a  circulatory 
system  of  a  more  highly  developed  type  than  in  any  of  the 
preceding  groups  of  animals.  The  ventricles  of  the  heart 
no  longer  communicate  with  each  other,  and  the  pure  and 
impure  blood  never  mingle.  Furthermore,  the  beating  of 
the  heart  is  comparatively  rapid,  rushing  the  oxygen  as 
fast  as  it  enters  the  blood  to  all  portions  of  the  body.  The 
result  is  that  everywhere  heat  is  being  generated,  so  neces- 
sary to  life  and  activity. 

In  the  lower  animals  no  special  means  are  employed  to 
husband  the  energy  thus  produced,  but  in  the  birds  the 
body  is  jacketed  in  a  non-conducting  coat  of  feathers  which 
prevents  its  dissipation.  For  this  and  other  reasons  the 
birds,  summer  and  winter,  maintain  an  even  and  relatively 
high  temperature  (102°-110°).  Like  the  mammals,  birds 
are  warm-blooded  animals,  full  of  energy,  restlessly  active 
to  an  extent  realized  in  few  of  the  cold-blooded  animals. 

193.  Digestive  system. — This  life,  at  high  pressure,  de- 
mands a  relatively  large  amount  of  food  to  make  good  the 
losses  due  to  oxidation.  The  appetites  of  some  growing 
birds  is  only  satiated  after  a  daily  meal  equal  to  from  one 
to  three  times  their  own  weight,  and  after  reaching  adult 
size  the  amount  of  daily  food  required  is  probably  not  less 
than  one-sixth  their  weight.  The  nature  of  the  food  is 
exceedingly  varied,  and  the  digestive  tract  and  certain  ac- 
cessory structures  are  obviously  modified  in  accordance 
with  it.  The  beak,  always  devoid  of  teeth  in  the  living 
form,  varies  extremely  according  to  the  work  it  must  per- 
form. The  same  is  true  of  the  tongue,  and  many  correlated 
modifications  exist  in  the  digestive  apparatus.  In  the 
birds  of  prey  and  the  larger  seed-eating  species,  such  as  the 


THE  BIRDS  207 

pigeons  and  the  domestic  fowls,  the  esophagus  dilates  into 
a  crop,  in  which  the  food  is  stored  and  softened  before  being 
acted  upon  by  the  gizzard.  The  latter  is  the  stomach,  pro- 
vided with  muscular  walls,  especially  powerful  in  the  seed- 
eaters,  and  with  an  internal  corrugated  and  horny  lining 
which,  in  the  absence  of  teeth,  serves  to  crush  the  food.  In 
some  species,  such  as  the  domestic  fowls  and  the  pigeons, 
this  process  is  aided  by  the  grinding  action  of  pebbles 
swallowed  along  with  the  food.  The  remaining  portions, 
with  pancreas  and  liver,  vary  chiefly  in  length,  and  are 
sufficiently  shown  in  Fig.  120  to  require  no  further  descrip- 
tion. 

194.  Nesting-habits.— A  few  birds,  such  as  the  ostriches 
and  terns,  merely  scoop  a  hollow  in  the  earth,  and  make  no 
further  pretense  of  constructing  a  nest.  On  the  other 
hand,  some  birds,  such  as  the  humming-birds  and  pewees, 
build  wonderful  creations  of  moss,  lichens,  and  spider-webs, 
lining  it  with  down,  and  concealing  it  so  skilfully  that 
they  are  not  often  found.  Every  bird  has  its  own  particular 
ideas  as  to  the  fitness  of  its  own  nest,  and  the  results  are 
remarkably  different,  and  form  an  interesting  feature  in 
studying  the  habits  of  birds.  Usually  the  female  takes 
upon  herself  the  choice  of  the  nest  and  its  construction ; 
but  these  duties  are  in  some  species  shared  by  the  male. 
After  the  eggs  are  laid,  the  male  may  also  aid  in  their 
incubation,  or  may  carry  food  to  the  female.  In  other 
species — for  example,  the  pigeons  and  many  sea-birds — the 
parents  take  turns  in  sitting  upon  the  eggs  and  in  the  sub- 
sequent care  of  the  young.  Finally,  there  are  certain  birds, 
such  as  the  cuckoo  and  cowbirds,  which  take  advantage  of 
the  industry  of  other  species  and  deposit  an  egg  or  two  in 
the  nests  of  the  latter.  All  the  work  of  incubation  and 
care  of  the  young  is  assumed  by  the  foster-parents,  which 
sometimes  neglect  their  own  offspring  in  their  desperate 
attempts  to  satisfy  the  appetites  of  the  rapidly  growing  and 
unwelcome  guests. 
36 


208  ANIMAL  FORMS 

The  eggs  of  birds  are  relatively  large,  and  are  often 
delicately  colored.  In  some  species  the  blotches  and  streaks 
of  different  shades  are  probably  protective,  as  in  the  plovers 
and  sandpipers,  whose  eggs  blend  perfectly  with  their  sur- 
roundings, but  many  other  cases  exist  not  subject  to  such 
an  explanation. 

The  young  require  a  high  degree  of  heat  for  their  devel- 
opment, and  this  is  usually  supplied  by  the  parent.  In  a 
very  general  way  the  length  of  sitting,  or  incubation,  is 
proportional  to  the  size  of  the  egg,  being  from  eleven  to 
fourteen  days  in  the  smaller  species,  to  seven  or  eight  weeks 
in  the  ostriches.  Before  hatching,  a  sharp  spine  develops 
on  the  beak,  and  with  this  the  young  bird  breaks  its  way 
through  the  shell.  Among  the  quails,  pheasants,  plovers, 
and  many  other  species,  the  young  are  born  with  a  covering 
of  feathers,  wide-open  eyes,  and  the  ability  to  follow  their 
parents  or  to  make  their  own  way  in  the  world.  Such 
nestlings  are  said  to  be  precocial,  in  distinction  to  the  alincal 
young  of  the  more  highly  specialized  species,  such  as  the 
sparrows,  woodpeckers,  doves,  birds  of  prey,  and  their  allies, 
which  are  born  helpless  and  depend  for  a  considerable  time 
on  the  parents  for  support. 

Some  of  the  owls,  crows,  woodpeckers,  sparrows,  quails, 
etc.,  remain  in  the  same  localities  where  they  are  bred. 
They  are  resident  birds.  Most  kinds  of  birds,  at  the  ap- 
proach of  winter,  migrate  toward  the  southern  warmer 
climes,  some  species  traveling  in  great  flocks,  by  day  or 
night,  and  often  at  immense  heights.  In  some  cases  this 
movement  appears  to  be  directly  related  to  the  food-supply  ; 
but  there  are  many  apparent  exceptions  to  such  a  theory, 
and  it  is  possible  that  many  birds  migrate  for  other  reasons. 
Certain  species  migrate  thousands  of  miles,  along  fairly 
definite  routes,  the  young,  sometimes  at  least,  guided  by 
the  parents,  which  in  turn  appear  to  remember  certain 
landmarks  observed  the  year  before.  Sea-birds,  in  their 
journeys  northward  or  southward,  keep  alongshore,  occa- 


THE  BIRDS  209 

sionally  veering  in  to  get  their  bearings  or  to  rest,  espe- 
cially in  the  presence  of  fogs. 

195.  Classification. — Most  zoologists  make  two  primary 
divisions  of  the  living  types  of  birds — those  like  the  ostrich 
with  flat  breast-bones,  and  the  other  the  ordinary  birds,  in 
which  the  breast-bone  has  a  strong  keel  for  the  attachment 
of  the  powerful  muscles  used  in  flight.     This  distinction  is 
not  of  high  importance,  but  we  may  use  it  as  a  convenience 
in  the  description  of  a  few  typical  forms  belonging  to  sev- 
eral orders  into  which  these  two  divisions  are  subdivided. 

196.  The  ostriches,  etc.  (Ratitse).— From   specimens  in- 
troduced or  from  pictures  we  are  doubtless  familiar  with 
the  ostriches  and  with  some  of  their  relatives.    The  African 
ostrich  (Struthio  camelus,  Fig.  121)  is  the  largest  of  living 
birds,  attaining  a  height  of  over  seven  feet,  and  is  further 
characterized  by  a  naked  head  and  neck,  two  toes,  and 
fluffy,  plume-like  feathers  over  parts  of  the  body.     They 
are  natives  of  the  plains  and  deserts  of,  Africa,  where  they 
travel  in  companies,  several  hens  accompanying  the  male. 
When  alarmed,  they  usually  escape  by  running  with  a  swift- 
ness greater  than  that  of  the  horse,  but  if  cornered  they 
defend  themselves  with   great   vigor  by  means   of  their 
powerful  legs  and  beaks.     Their  food  consists  of  insects, 
leaves,  and  grass,  to  which  is  added  sand  and  stones  for 
grinding  the  food,  as  in  the  domestic  fowl.     The  American 
ostriches  or  rheas,  are  smaller  ostrich-like  birds,  living  on 
the  plains  of  South  America.     Their  habits  are  essentially 
the  same  as  those  of  the  African  species. 

197.  The  loons,  grebes,  and  auks  (Pygopodes).— The  birds 
in  this  and  some  of  the  following  orders  are  aquatic  in 
their  habits.     All  have  broad,  boat-like  bodies,  which,  with 
the  thick  covering  of  oily  feathers,  enables  them  to  float 
without  effort.     The  legs  are  usually  placed  far  back  on 
the  body — a  most  favorable  place  for  swimming,  but  it  ren- 
ders such  birds  extremely  awkward  on  land.      The  grebes 
are  preeminently  water-birds.   The  pied-billed  grebe  or  dab- 


FIG.  121.— African  or  two-toed  ostrich  (Struthio  camelus).    Photograph  by  WIL- 
LIAM GKAHAM. 


THE  BIRDS  211 

chick  (Podilymbuspodiceps),  for  example,  found  abundantly 
on  the  larger  lakes  and  streams  throughout  the  United 
States,  captures  its  food,  sleeps,  and  breeds  without  leaving 
the  water.  The  loons  living  in  the  same  situations  as  the 
dabchick  are  also  remarkable  swimmers  and  divers.  Of 
the  three  species  found  in  this  country,  the  common  loon 
or  diver  (  Gavia  imber)  attains  a  length  of  three  feet,  and  is 
otherwise  distinguished  by  its  black  plumage,  mottled  and 
barred  with  white,  which  is  also  the  color  of  the  under 
parts.  The  auks,  murres  (see  frontispiece),  and  puffins  are 
marine,  and,  like  their  inland  relatives,  are  expert  swim- 
mers and  divers,  strong  fliers,  and  spend  much  of  their 
time  on  the  open  sea.  During  the  breeding-season  they 
assemble  in  vast  numbers  on  rugged  cliffs  along  the  shore, 
and  lay  their  eggs  on  the  bare  rock  or  in  rudely  constructed 
nests. 

198.  The  gulls,  terns,  petrels,  and  albatrosses  (Longi- 
pennes). — The  birds  belonging  to  this  group  are  among  the 
most  abundant  along  the  seacoast,  and  several  species  make 
their  way  inland,  where  they  often  breed.  All  are  char- 
acterized by  long,  pointed  wings  and  pigeon  or  swallow-like 
bodies,  which  are  carried  horizontally  as  the  bird  waddles 
along  when  ashore.  Many  are  excellent  swimmers  and 
powerful  fliers,  especially  the  petrels  and  albatrosses,  which 
sometimes  travel  hundreds  of  miles  at  a  single  flight. 

The  gulls  are  abundantly  represented  along  our  coasts, 
where  they  frequently  associate  in  companies,  usually  rest- 
ing lightly  on  the  surface  of  the  water,  or  wheeling  lazily 
through  the  air  on  the  lookout  for  food.  The  terns  are 
of  lighter  build  than  the  gulls  and  are  more  coastwise  in 
their  habits,  and  are  further  distinguished  by  plunging  like 
a  kingfisher  for  the  fishes  on  which  they  live.  Both  the 
gulls  and  terns  breed  in  colonies,  every  available  spot  over 
acres  of  territory  being  occupied  by  their  nests,  which  are 
usually  built  of  grass  and  weeds  placed  on  the  ground. 

The  petrels  and  albatrosses  are  at  home  on  the  high 


212 


ANIMAL  FORMS 


seas,  rarely  coming  ashore  except  at  the  breeding-season. 
Some  species  of  the  former  are  abundant  off  our  shores? 
especially  the  stormy  petrel  (Procellaria  pelagica)  or  Mother 
Carey's  chickens  ( Oceanites  oceanicus), which  are  often  seen 
winging  their  tireless  flight  in  the  wake  of  ocean  vessels. 
Among  the  dozen  or  so  albatrosses  few  reach  our  shores. 
The  wandering  albatross  (Diomedea  exulans),  celebrated  in 
story  and  as  the  largest  sea-bird  (fourteen  feet  between  the 
tips  of  its  outstretched  wings),  is  an  inhabitant  of  the 
southern  hemisphere,  and  only  rarely  extends  its  journeys 
to  more  northern  regions. 

199.  Cormorants  and  pelicans  (Steganopodes). — The  cor- 
morants and  pelicans  are  comparatively  large  water-birds 


FIG.  122.— White  pelicans  (P.  erythrorhynchus)  and  whooping-crane  (Grus  ameri- 
cana).    Photograph  by  W.  K.  FISHER. 

usually  abundant  along  the  seashore  and  in  many  sections 
of  the  United  States.     The  cormorants  or  shags  are  glossy 


THE  BIRDS  213 

black  in  color,  with  hooked  bills,  long  necks,  and  short 
wings,  which  give  them  a  duck-like  flight.  The  much 
larger  pelicans  (Fig.  122)  are  at  once  distinguished  by  long 
bills,  from  which  is  suspended  a  capacious  membranous  sac. 
All  these  birds  are  sociable  in  their  habits,  breeding,  roost- 
ing, and  fishing  in  great  flocks.  Their  food  consists  of 
fishes,  which  the  shags  pursue  under  water  and  capture  in 
their  hooked  beaks ;  while  the  pelicans,  diving  from  a  con- 
siderable height  or  swimming  rapidly  on  the  surface,  use 
their  pouches  as  dip-nets.  The  nests,  usually  built  of  sea- 
weed or  of  sticks,  are  placed  on  rocky  cliffs  or  on  the 
ground  in  less  elevated  places. 

200.  Ducks,  geese,  and  swans  (Lamellirostres). — The  birds 
of  this  order,  with  their  broad,  flat,  serrated  beaks,  short 
legs,  and  webbed  feet,  are  well  known,  for  in  a  wild  or 
domesticated  state  they  extend  all  over  the  earth.  All  are 
excellent  swimmers,  many  dive  remarkably  well,  and  are 
strong  on  the  wing.  While  a  considerable  number  breed 
within  the  United  States,  their  nesting-grounds  are  gener- 
ally farther  north,  and  in  the  early  spring  it  is  not  unusual 
to  see  them  migrating  in  flocks  from  their  warmer  winter 
homes.  Among  the  ducks,  the  mergansers,  mallards  (from 
which  our  domestic  species  have  been  derived),  the  teals, 
and  the  beautiful  wood-duck  remain  with  us  the  year 
round,  dwelling  on  quiet  streams  and  shallow  ponds,  living 
on  fish,  Crustacea,  and  seeds.  In  the  more  open  waters  of  the 
larger  lakes  and  along  the  seacoast  we  find  the  canvasback, 
the  scaup-ducks,  and  the  eiders  (Fig.  123)  which  supply  the 
famous  down  of  commerce.  Of  the  few  species  of  geese 
which  inhabit  the  United  States,  the  Canada  goose  (Branta 
canadensis)  is  perhaps  the  most  familiar.  During  their 
migrations  to  the  nesting  sites  they  fly  in  V-shaped  flocks, 
their  "  honks  "  announcing  the  opening  of  spring.  The 
brant  (B.  lernida)  is  also  common  in  the  eastern  part  of 
the  country,  where  it,  like  its  relations,  lives  on  vegetable 
substances  entirely.  The  swans  are  familiar  in  their  semi- 


FIG.  123.— American  eider-duck  (Somateria  dresseri). 


THE  BIRDS  215 

domesticated  state,  but  the  two  beautiful  wild  swans  found 
in  this  country  are  rarely  seen. 

201.  The  herons  and  bitterns  (Herodines). — The  herons 
and  bitterns  are  also  aquatic  in  their  habits,  but,  unlike  the 
swimming-birds,  they  seek  their  food  by  wading.    Adapting 
them  for  such  an  existence,  the  legs  and  neck  are  usually 
very  long,  and  the  bill,  longer  than  the  head,  is  sharp  and 
slender.     Among  the  relatively  few  species  in  the  United 
States,  the  great  blue  heron  (Ardea  herodias)  is  widely  dis- 
tributed, and  may  often  be  seen  standing  motionless  in 
some  shallow  stream  on  the  lookout  for   fish,  or  it  may 
wander  away  into  the  meadows  and  uplands  to  vary  its  diet 
with  frogs  and  small  mammals.     Even  more  familiar  is  the 
little  green  heron  or  poke  (Ardea  virescens),  which  also  is 
seen  widely  over  the  country.     The  night-herons,  as  their 
name  indicates,  stalk  their  prey  by  night,  and  during  the 
day  roost  in  companies — a  characteristic  common  to  most 
herons.     The  bitterns  or  stake-drivers  are  at  home  in  reedy 
swamps,  where  they  live  singly  or  in  pairs,  and  throughout 
the  night,  during  times  of  migration,  utter  a  booming  noise 
resembling  the  driving  of  a  stake  into  boggy  ground.     As 
a  rule,  the  herons  breed  as  they  roost — in  companies — build- 
ing bulky  platforms,  usually  in  trees.     The  bitterns,  on  the 
other  hand,  secrete  their  nests  on  the  ground  in  the  rushes 
of  their  marshy  home. 

202.  Cranes,  rails,  and  coots  (Paludicolae).— In  their  ex- 
ternal form  the  cranes  and  rails  resemble  the  herons,  but 
in  their   internal    organization   they   differ    considerably. 
They   likewise    inhabit   marshy  lands,  but   usually   avoid 
wading,  picking  up  the  frogs,  fish,  and  insects  or  plants 
along  the  shore  or  from  the  surface  of  the  water.    The  cranes 
are  comparatively  rare  in  this  country,  yet  one  may  occasion- 
ally meet  with  the  whooping-cranes  ( Grus  americana)  and 
sand-hill  cranes  (Grus  mexicand),  especially  in  the  South 
and  West.     They  are  said  to  mate  for  life,  and  annually 
repair  to  the  same  breeding- grounds,  where  they  build  their 


216  ANIMAL  FORMS 

nests  of  grass  and  weeds  on  the  ground  in  marshy  places. 
The  rails  are  more  abundant,  though  rarely  seen  on  ac- 
count of  their  habit  of  skulking  through  the  swamp 
grasses.  Only  rarely  do  they  take  to  the  wing,  and  then 
fly  but  a  short  distance,  with  their  legs  dangling  awk- 
wardly. Closely  related  to  them  are  the  coots  or  mud-hens 
(Fulica  americana),  which  may  be  distinguished,  however, 
by  their  slaty  color,  white  bills,  and  lobed  webs  on  the  toes, 
and  consequent  ability  to  swim.  All  over  the  United 
States  they  may  be  seen  resting  cm  the  shores  of  lakes  or 
quiet  streams,  or  swimming  on  the  surface  gathering  food. 
The  nest  consists  of  a  mass  of  floating  reeds,  which  the 
young  abandon  almost  as  soon  as  hatched. 

203.  The  snipes,  sandpipers,  and  plovers  (Limicolae), — The 
snipes,  sandpipers,  and  plovers  are  usually  small  birds, 
widely  scattered  throughout  the  country  wherever  there 
are  sandy  shores  and  marshes.  In  most  species  the  legs 
are  long,  and  in  connection  with  the  slender,  sensitive  bill 
fit  the  bird  for  picking  up  small  animals  in  shallow  water 
or  probing  for  them  deep  in  the  mud.  During  the  greater 
part  of  the  year  they  travel  in  flocks,  but  at  the  nesting- 
season  disperse  in  pairs  and  build  their  nests  in  shal- 
low depressions  in  the  earth.  The  eggs  are  usually 
streaked  and  spotted,  in  harmony  with  their  surroundings, 
as  are  the  young,  which  leave  the  nest  almost  as  soon  as 
hatched. 

Fully  fifty  species  of  these  shore-birds  live  within  the 
confines  of  the  United  States.  Among  these  the  woodcock 
(Philohela  minor)  and  snipe  ( Gallinago  delicata)  are  abun- 
dant in  many  places  inland,  where  they  probe  the  moist  soil 
for  food,  and  in  turn  are  eagerly  sought  by  the  sportsman. 
Even  more  familiar  are  the  sandpipers  and  plovers,  which 
are  especially  common  along  the  seacoast,  and  are  also 
abundantly  represented  by  several  species  far  inshore. 
Among  the  latter  are  the  well-known  spotted  sandpiper  or 
"  tip-up  "  (Actitis  macularia)  and  the  killdeer  plover 


THE  BIRDS 


217 


alitis  vocifera),  which   inhabit  the   shores   of  lakes  and 
streams  throughout  the  country. 

204.  Quail,  pheasants,  grouse,  and  turkey s(Gallinse).— The 
quail,  grouse,  and  our  domestic  fowls  are  all  essentially 


(Lophortyx  californicus).    Two- thirds  natural  size. 


ground-birds,  and  their  structure  well  adapts  them  to  such 
a  life.  The  body  is  thick-set,  the  head  small,  and  the  beak 
heavy  for  picking  open  and  crushing  the  seeds  and  berries 


218  ANIMAL  FORMS 

upon  which  they  live.  The  legs  and  feet  are  stout,  and 
fitted  for  scratching  or  for  running  through  grass  and 
underbrush.  Protective  colors  also  prevent  detection,  hut 
if  close  pressed  they  rise  into  the  air  with  a  rapid  whirring 
of  their  stubby  wings,  and  after  a  short  flight  settle  to  the 
ground  again.  During  the  breeding-season  the  male  usu- 
ally mates  with  a  number  of  hens,  which  build  rough  nests 
in  hollows  in  the  ground,  where  they  lay  numerous  eggs. 
The  young  are  precocial. 

The  quail  or  bob-white  (Colinus  virginianus)  and  the 
ruffed  grouse  (Bonasa  umbellus)  occur  throughout  the 
Eastern  States.  Over  the  same  area  the  wild  turkey 
(Meleagris  gallopavo)  once  extended,  but  is  now  almost 
extinct.  The  prairies  of  the  middle  West  support  the 
prairie-hen  (Tympanuchus  americanus),  and  the  valleys 
and  mountains  of  the  far  West  are  the  home  of  several 
species  of  quails,  some  of  which  are  beautifully  crested. 

205.  Pigeons  and  doves  (Columbse). — The  pigeons  and 
doves  belong  to  a  small  yet  well-defined  order,  with  upward 
of  a  dozen  representatives  in  the  United  States.  They  are 
of  medium  size,  with  small  head,  short  neck  and  legs,  and 
among  other  distinguishing  characters  frequently  possess  a 
swollen,  fleshy  pad  in  which  the  nostrils  are  placed.  In 
former  years  the  passenger-pigeon  (Ectopistes  migrator  ius), 
inhabiting  eastern  North  America,  was  probably  the  most 
common  species  in  this  country.  Their  flocks  contained 
thousands,  at  times  millions,  of  individuals,  which  often 
traveled  hundreds  of  miles  a  day  in  search  of  food,  to  return 
at  night  to  definite  roosts — a  trait  which  enabled  the  hunter 
to  practically  exterminate  them.  At  present  the  mourning- 
or  turtle-dove  (Zenaidura  macroura)  is  the  most  familiar 
and  wide-spread  of  the  wild  forms.  The  domestic  pigeons 
are  all  descendants  of  the  common  rock-dove  (Columba 
livid)  of  Europe,  the  numerous  varieties  such  as  the  tum- 
blers, fantails,  pouters,  etc.,  being  the  product  of  man's 
careful  selection.  In  the  construction  of  the  nest,  usually 


THE  BIRDS  219 

a  rude  platform  of  twigs,  and  in  the  care  of  the  young 
both  parents  have  a  share.  The  young  at  hatching  are 
blind,  naked,  and  perfectly  helpless,  and  are  fed  masticated 
food  from  the  crops  of  the  parents  until  able  to  subsist  on 
fruits  and  seeds. 

206.  Eagles,  hawks,  owls,  etc.  (Raptores). — The  birds  of 
prey,  all  of  which  belong  to  this  order,  are  carnivorous, 
often  of  large  size  and  great  strength,  and  are  widely  dis- 
tributed throughout  this  country.  The  vultures  live  on 
carrion,  some  of  the  small  hawks  and  owls  on  insects,  while 
the  majority  capture  small  birds  and  mammals  by  the  aid  of 
powerful  talons.  In  every  case  the  beak  is  hooked,  and  the 
perfection  of  the  organs  of  sight  and  hearing  is  unequaled 
by  any  other  animal,  man  included.  They  live  in  pairs, 
and  in  many  species  mate  for  life.  As  a  rule,  the  female 
incubates  the  eggs,  and  the  male  assists  in  collecting 
food. 

Among  the  vultures,  the  turkey-buzzard  (Cathartes  aura) 
is  most  abundant  throughout  the  United  States,  especially 
in  the  warmer  portions,  where  it  plays  an  important  part 
as  a  scavenger.  Of  the  several  species  of  hawks,  the  white- 
rumped  marsh-hawk  (Circus  huclsonius),  the  red-tailed 
hawk  (Buteo  lorealis),  the  red-shouldered  hawk  (Buteo 
lineatus),  and  above  all  the  bold  though  diminutive  spar- 
row-hawk (Falco  sparverius)  are  the  most  abundant  and 
familiar.  In  the  more  unsettled  regions  live  the  golden 
eagle  (Aquila  chrysaetus)  and  bald  eagle  (Haliaetus  leuco- 
cepJialus).  The  owls  are  nocturnal,  and  not  so  often  seen 
as  the  other  birds  of  prey,  yet  the  handsome  and  fierce 
barn  or  monkey-faced  :>wl  (Strix pratincold),  and  the  larger 
species,  such  as  the  great  gray  owl  (Scotiaptex  cinereua), 
and  the  beautiful  snowy  owl  (Nyctea  nyctea},  are  more  or 
less  common,  and  occasionally  seen.  Much  more  abundant 
is  the  little  screech-owl  (Megascops  asio),  and  in  the  West- 
ern States  the  burrowing-owl  (Speotyto  cunicularia],  which 
lives  in  the  burrows  of  the  ground-squirrels  and  prairie- 


220 


ANIMAL  FORMS 


dogs.    Fiercest  and  strongest -of  the  tribe  is  the  great 
horned  owl  (Bubo  virginianus). 


FIG.  125. — Golden  eagle  (Aquila  chrysaetus). 

207.  Cuckoos  and  kingfishers  (Coccyges).— Omitting  the 
order  of  parrots  (Psittaci),  whose  sole  representative  in  this 
country  is  the  almost  exterminated  Carolina  parrakeet 


THE  BIRDS  221 

(Conurus  caroUnensis),  we  next  arrive  at  the  cuckoos  and 
kingfishers,  which  differ  widely  in  their  habits.  The  black- 
or  yellow-billed  cuckoos  or  rain-crows  are  shy,  retiring 
birds,  with  drab  plumage,  and  though  seldom  seen  are  often 
fairly  abundant,  and  are  of  much  service  in  destroying 
insects.  Unlike  their  shiftless  European  relatives,  which 
lay  their  eggs  in  the  nests  of  others  birds,  they  build  their 
own  airy  homes  in  some  bush  or  hedgerow,  and  raise  their 
brood  with  tender  care.  The  belted  kingfisher  (Ceryle 
alcyori)  is  also  of  a  retiring  disposition,  and  spends  much 
of  its  time  on  some  branch  overlooking  the  water,  occa- 
sionally varying  the  monotony  by  dashing  after  a  fish,  or 
flying  with  rattling  cry  to  another  locality.  Their  nests 
are  built  in  holes  in  banks,  and  six  or  eight  young  are 
annually  reared. 

208.  The  woodpeckers  (Pici). — The  woodpeckers  are 
widely  distributed  throughout  the  world,  and  are  preemi- 
nently fitted  for  an  arboreal  life.  The  beak  is  stout  for 
chiseling  open  the  burrows  of  wood-boring  insects,  which  are 
extracted  by  the  long  and  greatly  protrusible  tongue.  The 
feet,  with  two  toes  directed  forward  and  two  backward,  are 
adapted  for  clinging,  and  the  stiff  feathers  of  the  tail  serve 
to  support  the  bird  when  resting.  Almost  all  are  bright- 
colored,  with  red  spots  on  the  head,  at  least  in  the  males, 
which  may  further  attract  their  mates  by  beating  a  lively 
tattoo  with  their  beaks  on  some  dry  limb.  The  glossy 
white  eggs  are  laid  in  holes  in  trees,  and  both  parents  are 
said  to  share  the  duties  of  incubation  and  feeding  the 
young.  Among  the  more  abundant  and  well-known  species 
is  the  yellowhammer  or  flicker  (Colaptes  auratus),  which 
extends  throughout  the  United  States.  Somewhat  less 
widely  distributed  is  the  red-headed  woodpecker  (Melaner- 
pes  erythrocephalus),  and  the  small  black-and-white  downy 
woodpecker  (Dryolates  pubescens).  This  is  often  called 
sapsucker,  but  incorrectly  so,  as,  like  all  but  one  of  our  other 
woodpeckers  it  feeds  on  insects.  The  yellow-bellied  wood- 


222 


ANIMAL  FORMS 


pecker  (Spliyrapicus  varius)  is  a  real  sapsucker,  living  on 
the  juices  of  trees.  A  close  relative  of  the  red-headed 
woodpecker,  the  California  woodpecker  (Melanerpes  formi- 
civorus),  is  renowned  for  its  habit  of  boring  holes  in  bark 
and  inserting  the  acorns  of  the  live  oak.  Subsequently  the 
bird  returns,  and  breaking  open  the  acorns,  devours  the 
grubs  which  have  infested  them,  and  apparently  eats  the 
acorns  also. 

209.  Swifts,  humming-birds,  etc.  (Macrochires).—  The  birds 
of  this  order  are  rapid,  skilful  fliers,  and  their  wings  are 
very  long  and  pointed.  The  feet,  on  the  other  hand,  are 


•! 


Fro.  126.— Night-hawk  (CJiordeiles  mrglnlanus)  on  nest.    Photograph  by  H.  K.  JOB. 

small,  relatively  feeble,  and  adapted  for  perching  or  cling- 
ing. Accordingly,  the  insects  upon  which  they  feed  are 
taken  during  flight  by  means  of  their  open  beaks.  The 
night-hawk  (CJiordeiles  virginianus),  roosting  lengthwise  on 
a  branch  by  day,  at  nightfall  takes  to  the  wing,  and  high 
in  the  air  pursues  its  food  after  the  fashion  of  a  swallow. 
In  the  same  haunts  throughout  the  United  States  the  whip- 


THE  BIRDS  223 


poorwill  (Antrostomus  vociferus)  occurs,  sleeping  by  day, 
but  active  at  night.  Neither  of  these  birds  constructs  nests, 
but  lays  its  streaked  and  mottled  eggs  directly  on  the 
ground.  The  chimney- swifts  (Chcetura pelagica),  swallow- 
like  in  general  form  and  habits,  but  very  unlike  the  swallows 


FIG.  127.— Anna  hummers  (one  day  old),  showing  short  bill  and  small  size  of  body. 
Compare  with  last  joint  of  little  finger. 

in  structure,  frequent  hollow  trees  or  unused  chimneys,  to 
which  they  attach  their  shallow  nests.  The  nearly  related 
humming-birds  are  chiefly  natives  of  tropical  America,  only 
a  few  species  extending  into  the  United  States.  Of  these 
the  little,  brilliantly  colored,  and  pugnacious  ruby  throat 
(Trochilus  colubris)  is  the  most  widely  distributed.  Its 
nest,  like  that  of  other  hummers,  is  composed  of  moss  and 
lichens  bound  together  with  cobweb  and  lined  with  down. 

210.  Perching  birds  (Passeres). — The  remaining  birds, 
over  six  thousand  in  all,  belong  to  one  order,  the  Passeres 
or  perch ers.  They  are  characterized  by  great  activity, 
interesting  habits,  frequently  by  exquisite  powers  of  song, 
and  in  addition  to  several  other  structural  arrangements 
have  the  feet  adapted  for  perching.  Their  nesting  habits 
37 


224:  ANIMAL  FORMS 

differ  widely,  but  in  every  case  the  young  are  helpless  at 
the  time  of  hatching,  and  require  the  care  of  the  parents. 

The  perchers  constitute  the  greater  number  of  the  birds 
living  in  the  meadows  and  woods,  and  are  more  or  less 


FIG.  128.— Anna  hummer  (Calypte  anna)  on  nest. 

common,  and  consequently  familiar  everywhere.  Among 
the  families  into  which  the  order  is  divided  that  of  the  fly- 
catchers (Tyrannidce),  the  crows  and  jays  (Corvidce),  the 
orioles  and  blackbirds  (Icteridai),  the  finches  and  sparrows 
(Fringillidce))  the  swallows  (Hirundinidce),  the  warblers 
{MniotiltidcB),  the  thrushes, robins,  and  bluebirds  (Turdidce), 
are  the  more  familiar,  though  the  others  are  equally  inter- 
esting. 


CHAPTEK  XVIII 

THE    MAMMALS 

211.  General  characteristics. — The  mammals,  constituting 
the  last  and  highest  class  of  the  vertebrates,  comprise  such 
forms  as  the  opossum  and  kangaroo,  the  whales  and  por- 
poises, hoofed  and  clawed  animals,  the  monkeys  and  man. 
All  are  warm-blooded,  air-breathing  animals,  having  the 
skin  more  or  less  hairy.  The  young  are  born  alive,  except 
in  the  very  lowest  forms,  which  lay  eggs  like  reptiles,  and 
for  some  time  after  birth  are  nourished  by  milk  supplied 
from  the  mammary  glands  (hence  the  word  mammals)  of 
the  mother.  The  skeleton  is  firm,  the  skull  and  brain 
within  are  relatively  large,  and,  with  few  exceptions,  four 
limbs  are  present. 

Most  of  the  mammals  inhabit  dry  land.  A  number, 
however,  such  as  the  whales  and  seals,  are  aquatic ;  while 
others,  such  as  the  beavers,  muskrats,  etc.,  though  not 
especially  adapted  for  an  aquatic  life,  are,  nevertheless, 
active  swimmers,  and  spend  much  of  their  time  in  the 
water. 

Mammals  tend  to  associate  in  companies,  as  we  may 
witness  among  the  ground-squirrels,  prairie-dogs,  rats, 
mice,  and  the  seals  and  whales.  In  many  cases  they  band 
for  mutual  protection,  and  often  fight  desperately  for  one 
another.  Claws,  hoofs,  and  nails  are  efficient  weapons,  and 
spiny  hairs,  as  on  the  porcupines,  bony  plates,  such  as 
encircle  the  bodies  of  the  armadillos,  and  thick  skin  and 
hair,  serve  as  a  protection.  The  hair  is  also  frequently 
colored  to  harmonize  the  animal  with  its  surroundings. 

225 


226  ANIMAL  FORMS 

Some  rabbits  and  hares  in  the  far  north  don  a  white  coat 
in  the  winter  season. 

212.  Skeleton. — As    in  other  vertebrates,  the   external 
form  of  mammals  is  dependent  in  large  measure  upon  the 
internal    skeleton.      This   consists   of    relatively  compact 
bones,  the  cavities  of  which  are  filled  with  marrow.     Those 
forming  the  skull  are  firmly  united,  and,  as  in  other  verte- 
brates, afford  lodgment  for  several  organs  of  special  sense 
and  for  the  brain,  which,  like  that  of  the  birds,  completely 
fills  the  cavity  in  which  it  rests.     The  vertebral  column  to 
which  the  skull  is  attached  differs  considerably  in  length, 
but  it  invariably  gives  attachment  to  the  ribs,  and  to  the 
basal  girdles  supporting  one  or  two  pairs  of  limbs.     Gener- 
ally speaking,  the  number  of  bones  in  the  head  and  trunk 
of  all  mammals  is  the  -same,  so  the  variations  we  note  in 
the  species  about  us,  for  example,  are  simply  due  to  differ- 
ences of  shape  and  proportion.     As  we  are  aware,  there  is  a 
great  dissimilarity  between  the  length  of  the  neck  of  man 
and  that  of  the  giraffe,  yet  the  number  of  bones  in  each 
is  precisely  the  same.     On  the  other  hand,  the  variations 
occurring  in  the  limbs  are  often  due  to  the  actual  disap- 
pearance of  parts  of  the  skeleton.     Five  digits  in  hand 
and  foot  is  the  rule,  and  yet,  as  we  well  know,  the  horse 
walks  on  the  tip  of  its  middle  finger  and  toe,  the  others 
being  represented  by  small,  very  rudimentary,  splint  bones 
attached  far  up  the  leg.     The  even-hoofed  animals  walk  on 
two  digits,  two   smaller  hoofed  toes   being  often   plainly 
visible  a  short  distance  up  the  leg,  as  in  the  pig.     In  the 
whales  the  hind  limbs  have  completely  disappeared,  and  in 
the   seals,  where  the  fore  limbs  are  modified,  as   in  the 
whales,  into  flippers,  the  hind  limbs  show  many  signs  of 
degeneration. 

213.  Digestive  system. — Some   mammals,  such  as  man, 
monkeys,  and  pigs,  are  omnivorous ;  others,  like  the  cud- 
chewers  and  gnawers,  are  vegetarians ;    and   still   others, 
like  the  foxes,  weasels,  and  bears,  are   carnivorous.      In 


THE  MAMMALS  227 

every  case  the  food  substances  are  acted  on  by  a  digestive 
system  constructed  on  the  same  general  plan  as  that  in  man, 
yet  modified  according  to  the  specific  work  it  is  required  to 
perform.  The  teeth  especially  afford  a  valuable  indication 
of  the  animal's  feeding  habits,  and,  as  we  may  notice  later, 
are  also  of  much  value  in  classification.  They  consist  of 
incisors  used  in  biting,  canines  for  tearing,  and  premolars 
and  molars  for  crushing  and  grinding. 

The  remaining  portions  of  the  digestive  tract,  esopha- 
gus, stomach,  and  intestine,  with  their  appended  glands,  are 
usually  not  unlike  those  possessed  by  the  squirrel  (Fig.  1). 
The  chief  differences  are  in  the  size  of  the  various  regions. 
The  stomach,  for  example,  may  be  long  and  slender  or  of 
great  dimensions,  and  its  surface  may  further  be  increased 
by  several  lobes,  which  are  especially  well  developed  in  the 
ruminants  or  cud-chewers.  The  intestine,  relatively  longer 
in  the  mammals  than  in  any  other  class  of  vertebrates,  also 
exhibits  great  differences  in  length  and  size.  In  the  flesh- 
eating  species  its  length  is  about  three  or  four  times  the 
length  of  the  body,  while  in  the  ruminants  it  is  ten  or 
twelve  times  the  length  of  the  animal. 

214.  Nervous  system  and  sense-organs. — As  before  noted, 
the  nervous  system  of  mammals  is  characterized  by  the 
large  size  and  great  complexity  of  the  brain.  Even  in  the 
simpler  species  the  cerebral  hemispheres  (large  front  lobes 
of  the  brain,  Fig.  1)  are  well  developed,  and  in  the  higher 
forms  of  the  ascending  series  they  form  by  far  the  larger 
part  of  the  brain.  The  sense-organs  also  are  highly  de 
veloped,  and  are  constructed  and  located  much  as  they  are 
in  man.  The  greatest  variations  occur  in  the  eyes.  In 
some  of  the  burrowing  animals  they  are  usually  small,  and 
in  some  of  the  moles  and  mice  may  even  be  buried  beneath 
the  skin  and  very  rudimentary.  On  the  other  hand,  they 
are  large  and  highly  organized  in  nocturnal  animals ;  more 
so,  usually,  than  in  those  which  hunt  their  prey  by  day. 
The  ears  also  have  different  grades  of  perfection,  which 


228  ANIMAL  FORMS 

appear  to  be  correlated  with  the  habits  of  the  animal. 
Among  the  species  of  subterranean  habits  the  sense  of  hear- 
ing is  largely  deficient ;  but,  on  the  other  hand,  it  is  ex- 
ceedingly keen  in  the  ruminants,  and  enables  them  to  detect 
their  enemies  at  surprisingly  great  distances.  In  these 
creatures  the  outer  ears  are  of  large  size  and  great  mobility, 
and,  placed  as  they  are  on  the  top  of  the  head,  serve  to  con- 
centrate the  sound-waves  on  the  delicate  apparatus  within. 
In  the  mammals  the  sense  of  smell  reaches  its  highest  de- 
velopment, especially  among  the  carnivores  which  scent  their 
prey.  On  the  other  hand,  it  is  said  to  be  absent  in  the 
whales  and  very  deficient  in  the  seals.  The  sense  of  taste, 
closely  related  to  that  of  smell,  is  located  in  taste-buds  on 
the  tongue,  and  is  also  more  acute  than  in  any  other  class  of 
animals.  The  sense  of  touch,  located  over  the  surface  of 
the  body,  is  especially  delicate  on  the  tips  of  the  fingers, 
the  tongue,  and  lips,  which  often  bear  long  tactile  hairs, 
called  whiskers  or  vibrissce. 

215.  Mental  qualities,— Correlated  with  the  high  degree 
of  perfection  of  the  brain  and  sense-organs  the  mammals 
show  a  higher  degree  of  development  of  the  intellectual 
faculties  than  any  other  class  of  animals.     In  many  cases 
their  acts  are  instinctive,  and  not  the  result  of  previous 
training  and  experience.     Just  as  the  duck  hatched  in  an 
incubator  instinctively  takes  to  the  water  and  pecks  at  its 
food,  or  as  the  bee  builds  its  symmetrical  comb,  many  of 
the  mammals  perform  their  duties  day  by  day.     On  the 
other  hand  many  other  mammals  are  also  undoubtedly  in- 
telligent.    They  possess  the  faculty  of  memory ;  they  form 
ideas  and  draw  conclusions  ;  they  exhibit  anger,  hatred,  and 
self-sacrificing  devotion  for  their  companions  and  offspring 
that  is  different  from  that  in  man  only  in  degree  and  not 
in  kind.     In  fact,  intelligence  differs  from  instinct  primar- 
ily in  its  power  of  choice  among  lines  of  action. 

216.  Classification.— Of  the  eleven  orders  into  which  the 
mammals  have  been  divided  eight  are  represented  in  this 


FIG.  129.— Three-toed  sloth  (Bradypus).    About  one-tenth  natural  size. 


230  ANIMAL  FORMS 

country.     Of  the  other  three  the  first  (Monotremes)  and 
simplest  of  the  eleven  is  represented  by  the  duck-mole 


PIG.  130.— Australian  duck-mole  (Ornithorhynchus  paradoxus).     One-fifth  natural 


size. 


(Ornithorhynchus)  living   in   the  Australian    rivers.     Its 
general  appearance  and  mode  of  life  are    illustrated    in 


THE  MAMMALS 


231 


Fig.  130.  The  monotremes  are  the  only  mammals  which 
lay  eggs-  One  or  two  eggs  are  laid  in  a  carefully  con- 
structed nest  where  the  young  are  hatched.  Another  order 
(Edentata)  includes  a  number  of  South  and  Central  Ameri- 
can forms,  among  which  are  the  ant-eaters,  armadillos, 
and  tree-inhabiting  sloths  (Fig.  129).  Still  another  order 
(Sirenia)  includes  the  fish-shaped  marine  dugong  and  sea- 
cows  or  manatees,  of  which  one  species  is  found  occasion- 
ally on  the  Florida  coast.  The  remaining  orders  are  de- 
scribed in  the  succeeding  sections. 

217.    The    opossums  and  kangaroos  (Marsupialia). — The 
lowest  order  of  mammals  represented  in  the  United  States 


FIG.  131.— Opossum  (Didelphys  virginiand).     One- tenth  natural  size.    Photograph 
by  W.  H.  FISHER. 

is  that  of  the  marsupials.  It  includes  the  opossums  and 
kangaroos,  together  with  a  number  of  comparatively  small 
and  unfamiliar  animals  living  chiefly  in  and  about  Australia. 


232  ANIMAL  FORMS 

The  opossums,  fairly  abundant  throughout  the  warmer 
portions  of  this  country,  are  rat-like  creatures,  with  scaly 
tails,  yellowish-white  fur,  large  head,  and  pointed  snout. 
Except  at  the  breeding  season  they  lead  solitary  lives, 
sleeping  in  the  holes  of  trees  by  day  and  at  night  feeding 
on  roots,  birds,  and  fruits. 

The  kangaroos,  familiar  from  specimens  in  menageries 
or  museums,  chiefly  inhabit  the  plains  of  Australia.  The 
giant  gray  kangaroos  (Macropus  giganteus),  attaining  a 
height  of  over  six  feet,  go  in  herds,  and  owing  to  the  great 
development  of  their  hind  limbs  and  tails  are  able,  when 
alarmed,  to  travel  with  the  swiftness  of  a  horse.  Several 
smaller  species,  some  no  larger  than  rabbits,  live  among 
the  brush,  and  like  their  larger  relatives  crop  the  grass  and 
tender  herbage  with  sharp  incisor  teeth. 

While  the  marsupials  do  not  lay  eggs  as  does  the  duck- 
mole,  they  allow  them  to  develop  within  the  body  for  a 
very  short  time  only.  Hence  the  young,  when  born,  are 
scarcely  more  than  an  inch  in  length,  and  are  blind,  naked, 
and  perfectly  helpless.  At  once  they  are  placed  by  the 
mother  in  the  pouch  of  skin,  or  marsupium,  on  the  under 
side  of  her  body.  In  this  the  young  are  suckled  and  pro- 
tected until  able  to  gather  their  own  food  and  fight  their 
own  way. 

218.  Rodents  or  gnawers  (Glires). — The  rodents  are  a 
large  group  of  mammals,  including  such  forms  as  the  rats, 
mice,  squirrels,  gophers,  and  rabbits.  They  are  readily  dis- 
tinguished by  their  clawed  feet  adapted  for  climbing  or 
burrowing,  and  by  large  curved  incisor  teeth.  Unlike 
ordinary  teeth,  they  grow  continually,  and,  owing  to  the 
restriction  of  the  hard  enamel  to  their  front  surfaces,  wear 
away  behind  faster  than  in  front,  thus  producing  a  chisel- 
like  cutting  edge. 

The  largest  of  our  native  rodents  is  the  porcupine 
(Erethizon  dorsatus),  which  ranges  from  Maine  to  Mexico, 
and  attains  a  length  of  nearly  three  feet.  Many  of  the  hairs 


THE  MAMMALS  233 

of  the  body  have  the  form  of  stiff,  barbed  spines  (Fig.  132), 
readily  dislodged  so  that  the  animal  requires  no  other  wea_ 
pon  of  defense.  The  rabbits  and  hares  are  of  smaller  size, 
and  the  cottontails  especially  are  widely  distributed.  West 
of  the  Mississippi  the  jack-rabbits  are  familiar,  and  are 


Fro.  132.— Porcupine  (Hystrix  cristatd).    One-tenth  natural  size.— After  BREITM. 

famous  for  their  great  speed.  Like  the  porcupines,  they 
feed  on  leaves  and  grass,  and  are  often  very  destructive. 
The  mice,  especially  the  field  and  white-footed  mice,  are 
abundant  in  woodland  and  meadow  throughout  the  United 
States.  The  house-mouse  (Mus  musculus)  is  a  native  of 
Europe,  as  is  the  common  rat  (M.  decumanus),  which  was 
imported  over  a  century  ago.  The  wood-rat  (Neotoma)^ 
however,  is  native,  and  may  be  found  in  many  localities 
from  east  to  west.  The  muskrat  (Fiber  zibetliicus),  beaver 
(Castor  canadensis),  and  woodchuck  (Arctomys  monax)  were 
also  more  or  less  plentiful  formerly,  but  in  many  localities 
are  well-nigh  exterminated.  The  squirrels,  on  the  other 
hand,  continue  to  exist  in  large  numbers.  The  prairie- 


234  ANIMAL  FORMS 

dogs,  ground-squirrels,  and  chipmunks  of  the  terrestrial 
species  are  of  frequent  occurrence,  and  of  the  tree-dwellers 
the  fox,  gray  and  red  squirrels  are  well  known  in  many 
sections  of  the  United  States. 

219.  Insect-eating  mammals   (Insectivora). — The  shrews 
and  moles  belonging  to  this  order  are  representatives  of  a 
large   group   of    small  animals,  which,  unlike   the  major 
number  of  rodents,  live  on  insects.     The  shrews,  of  which 
there  are  several  species  in  this  country,  are  small,  mouse- 
like creatures,  nocturnal  in  their  habits,  and  hence  rarely 
seen.     The  moles  are  of  much  larger  size,  and  owing  to 
their  burrowing   proclivities   scarcely   ever  appear  above 
ground,  but  excavate  elaborate  burrows  with  their  shovel- 
like  feet,  devouring  the  insects  which  fall  in  their  way.    The 
common  mole  (Scalops  aquations)  extends  from  the  eastern 
seaboard  to  the  Mississippi  River,  where  it  is  replaced  by 
the  prairie-mole  (S.  argenteus),  which  extends  far  to  the 
west,  into  a  country  inhabited  by  other  species. 

220.  The  bats  (Cheiroptera).— The  bats  are  also  insectiv- 
orous, but  their  habits  are  widely  different  from  those  of 
the  shrews  and  moles.     The  forearm  and  the  fingers  of  the 
fore  limbs  are  greatly  elongated,  and  are  connected  by  a 
thin  papery  membrane,  which  also  includes  the  hind  limbs 
and  tail,  arid  serves  as  an  efficient  organ  of  flight.     During 
the  day  they  remain  suspended  head  downward  in  some 
dark   cranny,   awakening    at  nightfall  to   capture   flying 
insects.     Several   species   are  found  in  this  country,  the 
most  common  being  the  little  brown  bat  ( Vespertilio  fus- 
cus),  with  small,  fox-like  face,  large  erect  ears,  and  short 
olive-brown  hair.     The  red  bat  (Lasiurus  lorealis)  is  also 
plentiful  everywhere  throughout  the  United  States,  and  is 
distinguished  from  the  preceding  by  its  somewhat  larger 
size  and  long  reddish-brown  fur. 

221.  The  whales    and   porpoises    (Cete).— The    animals 
belonging  to  this  order,  the  whales  (Fig.  133),  porpoises,  and 
dolphins,  are  aquatic  animals  bearing  a  resemblance  to  fishes 


THE  MAMMALS 


235 


only  in  external  form.    The  cylindrical  body  has  no  distinct 
neck,  the  comparatively  large  head  uniting  directly  with 


the  cylindrical  body,  which  terminates  in  the  tail  with  hori- 
zontally placed  fins.     No  external  signs  of  hind  limbs  exist, 


236  ANIMAL  FORMS 

while  the  fore  limbs  are  short  and  capable  of  being  moved 
only  as  a  whole.  External  ears  are  also  absent.  The  eyes 
are  exceedingly  small,  those  of  individuals  attaining  a  length 
of  from  fifty  to  eighty  feet,  being  in  some  species,  at  least, 
but  little  larger  than  those  of  an  ox.  These  are  often  placed 
at  the  corners  of  the  mouth.  The  nasal  openings,  often 
known  as  blow-holes,  are  situated  on  the  forehead,  and  as 
the  whale  comes  to  the  surface  for  air  afford  an  outlet  for 
the  stream  of  breath  and  vapor  often  blown  high  in  the 
air — a  process  known  as  spouting.  In  some  of  the  whales, 
such  as  the  dolphin,  porpoise,  and  sperm-whales,  the  teeth 
persist  throughout  life,  but  in  most  of  the  larger  species 
they  never  "  cut  "  the  gum,  but  early  disappear,  and  their 
place  is  taken  by  large  numbers  of  whalebone  plates  with 
frayed  edges  which  act  as  strainers.  The  smaller-toothed 
forms  (porpoises,  dolphins,  and  several  species  of  grampus) 
are  frequently  seen  close  to  the  shore,  where  they  are  usu- 
ally actively  engaged  in  capturing  fish.  On  the  other  hand, 
the  larger  species,  such  as  the  humpback,  right  whale,  and 
sulfurbottom,  not  uncommon  along  our  coasts,  especially 
to  the  northward,  live  on  much  smaller  organisms.  With 
open  mouth  these  whales  swim  through  the  water  until 
they  collect  a  sufficient  quantity  of  jelly-fishes,  snails,  and 
Crustacea,  then  closing  the  mouth  strain  out  the  water 
through  the  whalebone  fringes  and  swallow  the  residue. 

As  noted  above,  the  animals  of  this  order  are  almost 
wholly  devoid  of  hair,  but  the  heat  of  the  body  is  retained 
by  a  thick  layer  of  fat  beneath  the  skin.  This  "  blubber  " 
also  gives  lightness  to  the  body  (as  do  the  voluminous  lungs), 
and,  furthermore,  yields  large  quantities  of  oil,  which  in 
former  times  made  "  whale-fishing  "  a  profitable  industry. 
The  whales  bear  one,  rarely  two  offspring,  which  are  solicit- 
ously attended  by  the  mother  for  a  long  time.  The  smaller 
species  grow  to  a  length  of  from  five  or  eight  feet  (por- 
poises, dolphins)  to  twice  this  size  (grampuses) ;  while  the 
larger  whales,  by  far  the  largest  of  animals,  range  from 


THE  MAMMALS  237 

thirty  to  over  a  hundred  feet  in  length  with  a  weight  of 
many  tons. 

222.  Hoofed  mammals  (Ungulata).— The  order  of  hoofed 
animals  or  ungulates  includes  a  large  number  of  forms  like 
the  zebra,  elephant,  hippopotamus,  giraffe,  deer,  and  several 
other  wild  species,  some  of  which  are  domesticated,  such 
as  horses,  sheep,  goats,  and  cattle.  All  of  these  animals 
walk  on  the  tips  of  their  toes,  and  the  claws  have  become 
developed  into  hoofs.  The  order  is  divided  into  the  odd- 
toed  forms  (perissodactyls),  such  as  the  rhinoceros  with 
three  toes  and  the  horse  with  one,  and  the  even-toed  (artio- 
dactyls),  as  the  pigs  with  four,  and  the  ox,  deer,  etc.,  with 
two  toes. .  The  even-toed  forms  are  again  divided  into 
those  which  chew  the  cud  (ruminants)  and  those  which  do 
not  (non-ruminants).  Xo  living  native  odd-toed  mammal 
exists  in  this  country,  and  of  the  wild  even-toed  species  all 
are  ruminants.  In  the  members  of  this  latter  group  the 
swallowed  food  passes  into  a  capacious  sac  (the  paunch),  is 
thoroughly  moistened,  and  passed  into  the  second  division 
(the  honeycomb),  later  to  be  regurgitated  and  ground  by 
the  powerful  molars.  It  is  then  reswallowed,  and  under- 
goes successive  treatment  in  the  other  two  divisions  of 
the  stomach  (the  manyplies  and  reed)  before  entering  the 
intestine. 

Among  the  North  American  ruminants,  the  deer  fam- 
ily ( Cervidce)  is  the  best  represented.  In  the  more  unsettled 
regions  of  the  East  the  red  deer  is  still  common,  and  the 
same  may  be  said  of  the  white-tailed,  black-tailed,  and 
mule-deer  of  the  West.  Among  the  woods  and  lakes  to 
the  northward  live  the  reindeer  and  caribou,  and  the  largest 
of  the  deer  family,  the  moose,  which  attains  the  size  of  the 
horse.  Of  nearly  the  same  size  is  the  wapiti  or  elk,  whose 
general  characters  are  shown  in  Fig.  134.  In  all  of  the 
above-mentioned  species  the  horns,  if  present,  are  confined 
to  the  male  (except  in  the  reindeer),  and  are  annually  shed 
after  the  breeding  season. 


THE  MAMMALS  239 

The  native  hollow-horned  ruminants  (Bovidce)  are  at 
present  confined  to  the  Western  plains,  and  comprise  the 
pronghorn  antelope  (Antilocapra  americana),  the  wary  big- 
horn or  Eocky  Mountain  sheep  (Ovis  canadensis),  living  in 
mountain  fastnesses,  and  the  buffalo  or  bison  (Bison  ameri- 
canus).  All  of  these  species  were  formerly  abundant, 
especially  the  pronghorn  and  buffalo,  which  roamed  the 
plains  by  thousands,  but  their  extermination  has  been 
nearly  complete,  small  herds  only  persisting  in  a  few  wild, 
inaccessible  regions,  or  protected  in  parks. 

Our  domestic  sheep  and  cattle  are  probably  the  descend- 
ants of  several  wild  species  living  in  Europe  and  other 
portions  of  the  world.  Of  the  domesticated  ungulates  the 
horse  is  the  direct  descendant  of  Asiatic  wild  breeds ;  while 
the  pig  traces  its  ancestry  back  to  the  wild  boar  (Sus  scrofa) 
of  Europe,  and  probably  a  native  species  (S.  indicus)  of 
eastern  Asia. 

223.  Flesh-eating  mammals  (Ferae). — The  order  of  Ferce 
or  Carnivora  is  typically  exemplified  by  such  animals  as  the 
lions,  tigers,  bears,  dogs,  cats,  and  seals,  forms  which  differ 
from  all  other  mammals  by  the  large  size  of  the  canine  teeth 
(often  called  dog-teeth)  and  the  molars,  which  are  adapted 
for  cutting,  not  crushing.  The  limbs,  terminated  by  four 
or  five  flexible  digits,  bear  well-developed  claws,  which,  to- 
gether with  the  teeth,  serve  for  tearing  the  prey.  While 
the  bears  shuffle  along  on  the  soles  of  their  feet,  the  greater 
number  of  species,  as  illustrated  by  the  dog  and  cat,  tread 
noiselessly  on  tiptoe.  Almost  all  are  fierce  and  bold,  with 
remarkably  keen  senses  and  quick  intelligence,  and  are  the 
dreaded  enemies  of  all  other  orders  of  mammals. 

The  largest  land-inhabiting  carnivora  are  the  bears,  of 
which  the  brown  or  cinnamon  bear  ( Ursus  americanus), 
inhabiting  North  America  generally  where  not  extermi. 
nated,  and  the  huge  grizzly  ( Ursus  horribilis)  of  the  West- 
ern mountains,  are  the  best-known  species.  The  former 
lives  on  berries  and  juicy  herbs,  while  the  grizzly  prefers 
38 


240 


ANIMAL  FORMS 


the  flesh  of  animals  which  it  kills.  The  raccoon  (Fig.  135) 
(Procyon  lotor)  is  found  in  wooded  districts  all  over  the 
United  States,  and  its  general  appearance  and  thieving 
propensities  are  well  known.  Almost  everything  is  accept- 


able as  an  article  of  food,  and  its  fondness  for  poultry  and 
vegetables  makes  it  an  unmitigated  nuisance.  The  otters, 
skunks,  badgers,  wolverenes,  sables,  minks,  and  weasels,  while 
differing  considerably  in  general  appearance  and  habits,  nev- 


THE  MAMMALS  241 

ertheless  belong  to  one  family  (the  weasel  family,  Mustelidce), 
and  are  more  or  less  valued  for  their  fur.  Almost  all  are 
characterized  by  a  fetid  odor,  especially  the  skunk,  which 
is  notoriously  offensive,  and  in  consequence  is  avoided  by 
all  other  animals. 

The  dog  family  is  represented  by  several  widely  distrib- 
uted varieties  of  the  red  fox  ( Vulpes  pennsylvanicus)  and 
gray  fox  ( Urocyon  cinereo-argentatus),  and  by  the  coyotes 


FIG.    136.— Silver   fox   (Vulpes  pennsylvanicus,   var.   argentatus).     Photograph 
by  W.  K.  FISHEK. 

(Canis  latrans)  and  wolves  (Canis  nubilus).  The  domestic 
dog  (Canis  familiaris)  is  probably  the  descendant  of  the 
wolf,  and  owing  to  man's  careful  breeding  during  thou- 
sands of  years  has  formed  several  widely  differing  varieties. 
The  cat  family,  comprising  the  most  powerful,  savage, 
and  keenest-scented  carnivora,  is  represented  by  the  lion, 
tiger,  jaguar,  and  hyena.  In  this  country  the  group  is 
represented  by  the  lynx  (Lynx  canadensis),  the  wildcat 
(Lynx  rufus),'  and  the  panther  or  puma  (Felis  concolor), 
which  attains  the  length  of  nearly  five  feet.  The  domestic 
cat  has,  like  the  dog,  been  domesticated  for  centuries,  and 
has  possibly  descended  from  an  African  species  (Felis 


FIG.  137.— Panthers  (Felis  concolor)  and  peccaries  (Dicotyles  torquatus). 


THE  MAMMALS 


243 


caff r a),  which  was  held  sacred  by  the  Egyptians,  who  em- 
balmed them  by  thousands. 

224:.  Man-like  mammals  (Primates).— The  last  and  high- 
est order  of  mammals,  the  Primates,  includes  the  lemurs, 
monkeys,  and  man.  The  first  of  these  are  strange  squir- 
rel-like forms  living  chiefly  in  trees  in  Madagascar  and 
neighboring  regions  where  they  feed  on  insects.  The  apes 
and  monkeys  are  divided  into  Old  and  New  World  forms, 
which  differ  widely  from  each  other.  The  American  species 
are  marked  by  flat  noses,  with  the  nostrils  far  apart.  All  are 
arboreal,  many  have  long  prehensile  tails,  and  the  digits  bear 
nails,  not  claws.  Among  them  are  several  species  of  marmo- 
sets, the  howling  monkeys  (Myocetes),  the  spider-monkeys 
(Ateles),  and  the  capuchins  (Cebus),  all  of  which  are  more  or 
less  common  in  captivity.  In  the  Old  World  apes,  on  the 
other  hand,  the  nostrils  are  close  together  and  are  directed 
downward,  the  tail  is  never 
prehensile,  and  in  some  cases 
is  rudimentary,and  may  even 
disappear.  The  lowest  spe- 
cies (the  dog-like  apes)  in- 
clude the  large,  clumsy  ba- 
boons, among  them  the  fa- 
miliar blue-nosed  mandrill 
(Cynocsphalus  maimon)  and 
several  other  species  of  light- 
er frame,  such  as  the  long- 
tailed  monkey  (Cercopithe- 
cus)  (Fig.  139),  the  tailless 
Macacus,  common  in  menag- 
eries, and  the  Barbary  ape,  in- 
habiting northern  Africa  and 

extending  Over  into  Spain.  FIG.  138.—  Baby  orang-utan.    From  life. 

The   remaining   anthro- 
poid or  man-like  apes  include  the  gibbons  (Hylolates),  orang- 
utan (Simia),  gorilla  (Gorilla),  and  chimpanzee  (Anthropo- 


FIG.  139. — A  monkey  ( Cercopithecus)  in  a  characteristic  attitude  of  watchfulness. 


THE  MAMMALS 


245 


pithecus).  The  gibbons,  inhabiting  southeastern  Asia,  pos- 
sess arms  of  such  length  that  they  are  able  to  touch  their 
hands  to  the  ground  as  they  stand  erect.  They  are  thus 
adapted  for  a  life  in  the  trees,  where  they  spend  most  of  their 
time  feeding  on  fruit,  leaves,  and  insects.  In  the  same  dis- 
trict the  orang  occurs,  walking  when  on  the  ground  on  its 
knuckles  and  the  sides  of  its  feet.  It  prefers  the  life  in 
the  trees,  however,  in 
which  it  builds  nests 
serving  for  rest  and 
concealment.  The  go- 
rilla (Fig.  140),  the 
largest  of  apes,  attain- 
ing a  height  of  over 
five  feet  and  a  weight 
of  two  hundred 
pounds,  is  a  native  of 
Africa,  where  it  lives 
in  families  and  sub- 
sists on  fruits.  The 
same  region  is  the 
home  of  the  chimpan- 
zee, which  in  its  vari- 
ous characteristics  ap- 
proaches most  nearly 
to  man. 

Man  (Homo  sapi- 
ens) is  distinguished 

by  the  inability  to  oppose  the  big  toe  as  he  does  his  thumb — 
a  feature  associated  with  his  erect  position — and  by  the  rela- 
tively enormous  size  of  the  brain.  Even  in  an  average  four- 
year-old  child  or  an  Australian  bushman  the  brain  is  twice  as 
large  as  in  the  gorilla.  With  this  relatively  great  develop- 
ment of  the  nervous  system  is  correlated  superior  mental 
faculties,  which  together  with  social  habits  and  powers  of 
speech  exalt  man  to  a  position  far  above  the  highest  ape. 


FIG.  140.— Gorilla  (Gorilla). 


246  ANIMAL  FORMS 

As  usually  understood,  the  family  of  man  (Hominidce) 
contains  but  a  single  species,  cosmopolitan  and  highly  vari- 
able. This  species  is  "now  split  up  into  many  subspe- 
cies or  races,  the  native  man  of  this  continent,  or  '  Ameri- 
can Indian,'  being  var.  americanus.  Other  races  now 
naturalized  in  America  are :  the  Caucasian  race,  var.  euro- 
pceus\  the  Mongolian  race,  var.  asiaticus\  and  the  negro 
race,  afer.  The  first  of  these  is  an  immigrant  from  Europe, 
the  second  from  Asia,  and  the  third  was  brought  hither 
from  Africa  by  representatives  of  var.  europceus  to  be  used 
as  slaves." 


CLASSIFICATION  OF  ANIMALS 

The  following  table  of  classification  is  designed  to  show  the  systematic  position 
of  the  more  important  animals  mentioned  in  the  text. 

ANIMAL  KINGDOM 

ONE-CELLED  ANIMALS: 

BRANCH  I.    PROTOZO'A  (protos,  first ;  zoon,  animal) 
Class  I.   Rhizop'oda  (rhiza,  root ;  pous,  foot). 

Amce'ba,  Difflu'gia. 

Class  II.   Infuso  ria  (organisms  found  in  infusions). 
Order  1.   Flagella'ta  (flagellum,  a  whip). 

Euglen'a,  Codosig'a,  Pandori'na,  Vol'vox. 
Order  2.   Cilia'ta  (cilium,  an  eyelash). 
Paramce'cium,  Vorticel'la. 

MANY-CELLED  ANIMALS  (Metazoa): 

BRANCH  II.  PORIF  ERA  (porus,  pore ;  fero,  to  carry) 
Class  I.  Forifera  (or  sponges). 

BRANCH  III.   COELENTERA  TA  (animals  with  combined  body  and 

stomach  cavity) 
Class  I.  Hydrozo'a  (hydra,  water-serpent ;  zoon,  animal). 

Hy'drOf  Gfonionemus,  Hydractin'ia,  Portuguese  man-of-war. 
Class  II.  Scyphozo'a  (scyphos,  cup ;  zoon,  animal). 

Rhlzos 'toma,  Halidys'tus. 
Class  III.  Actinozo'a  (actis,  a  ray ;  zoon,  animal). 

BRANCH  IV.  PLATYHELMIN'THES  *  (platus,  flat ;  helminthos, 

a  worm) 

Class  I.  Plato'da  (platus,  flat;  eidos,  likeness). 
Plana'ria,  Leptdpla'na. 

*  For  purposes  of  convenience,  the  flatworms  (Platyhelminthes), 
roundworms  (Nematelminthes),  and  segmented  worms  (Annelids)  are 
combined  in  one  branch  (The  Worms)  in  the  text. 

247 


248  ANIMAL  FORMS 

Class  II.    Tremato  da  (trematodes,   pierced   with  holes,   from   the 
erroneous  belief  that  the  suckers  are  holes  into  the  body). 

Liver  fluke,  Epidel'la. 
Class  III.   Cesto'da  (cestos,  a  girdle ;  eidos,  likeness). 

Tapeworm. 

BRANCH  V.  NEMATELMINTHES  (nema,  thread;  helminthos,  a 

worm) 
Class  I.  Nemato'da. 

Vinegar  eel  (Anguillula),  Trichl'na,  horsehair  snake  (Gordius). 

BRANCH  VI.  NEMERTIN  E A  (nemertes,  a  sea-nymph) 
BRANCH  VII.  ROTIF  ERA  (rota,  a  wheel;  fero,  to  carry) 

BRANCH  VIII.   ANNELIDA  (annelus,  a  ring) 
Class  I.   Cheet5poda  (chaite,  bristle;  pous,  foot). 
Order  1.   Polychae'te  (polus,  many ;  chaite,  bristle). 

Ner'eis,  Poly  no' e,  Ser'pula,  Sabel'la. 
Order  2.   Oligochae  te  (oligos,  few ;  chaite,  bristle). 

Earthworm  (Lum'lricus). 
Class  II.  Hirudin'ea  (hirudo,  a  leech). 

Leeches. 

Class  III.  Gephyrea  (gephura,  a  bridge,  because  these  animals  were 
once  supposed  to  bridge  the  gap  between  the  worms  and  sea- 
cucumbers). 

BRANCH  IX.  MOLLUSCOI  DA 

Class  I.  Pblyzo'a  (polus,  many ;  zoon,  animal — colonial  animals). 

Polyzoa,  sea-mats. 
Class  II.   Brachiop'oda  (brachion,  arm ;  pous,  foot). 

Lamp-shells  (brachiopods). 

BRANCH  X.   MOLLUS'CA  (mollis,  soft) 
Class  I.   Lamellibranchia'ta  (lamella,  a  plate ;  branchia,  gill). 

Clams,  mussels,  oysters,  ship-worm  (Tere'dd). 
Class  II.   Gastrbp'oda  (gaster,  stomach ;  pous,  foot). 

Snails,  slugs,  armadillo  snails,  naked  snails,  nudibranchs. 
Class  III.   Cephalbp'oda  (cephale,  head  ;  pous,  foot). 

Squids,  cuttlefishes,  devil-fishes  (Oc'topus),  nautilus. 

BRANCH   XI.  ECHINODER'MATA  (echinos,  a  hedgehog;  derma, 

skin) 

Class  I.   Asteroi'dea  (aster,  star;  eidos,  likeness). 
Starfishes. 


CLASSIFICATION  OF  ANIMALS  249 

Class  II.   Ophiuroi'dea  (ophis,  serpent;  oura,  tail;  eidos,  likeness). 

Serpent-  or  brittle-stars,  basket-stars. 

Class   III.  Holothuroi  dea   (holothurion,  a  kind  of  water  polyp; 
eidos,  likeness). 

Sea-cucumbers. 
Class  IV.   Crinoi  dea  (crinon,  lily ;  eidos,  likeness). 

Sea-lilies  or  crinoids. 
Class  V.   Echinoi  dea  (echinos,  hedgehog;  eidos,  likeness). 

Sea-urchins. 

BRANCH  XII.   ARTHROP  ODA  (arthron,  joint ;  pous,  foot) 
Class  I.   Orusta'cea  (crusta,  a  crust  or  shell). 

Fairy-shrimp   (Brdnchip'us),  water-fleas  (DapJi 'nia),  cop'epod, 
Cy' clops,  goose  barnacle,  acorn  barnacle,  Saccull'na,  opossum- 
shrimp,  prawn,  lobster,  crayfish,  cancer-crab,  rock-crab,  pill- 
bug  or  i'sopod,  sand-hopper  or  amphi'pod. 
Class  II.   Onyc5ph'ora  (onyx,  claw ;  pJiero,  to  carry). 

Pertp'atus). 
Class  III.   Myriop  oda  (myrios,  numberless;  pous,  foot). 

Cent'iped,  thousand-legs. 

Class  IV.  Insec'ta  (insectum,  cut  in,  owing  to  the  grooves  surround- 
ing the  body). 

Fishmoth,  springtail,  cockroach,  grasshopper,  cricket,  katydid, 
locust,  dragon-fly,  caddis-fly,  may-fly,  white  ants  or  termites, 
ant-lion,  water-boatman,  water-bug,  back-swimmer,  chinch- 
bug,  squash-bug,  lice,  plant-lice,  Phyllox' era,  scale-insect, 
gnat,  mosquito,  flea,  house-fly,  stag-beetle,  wood-beetle,  water- 
beetle,  potato-beetle,  ladybug,  firefly,  moth,  butterfly,  ants, 
honey-bees  and  bumblebees,  wasps,  hornets,  yellow-jackets. 
Class  V.  Arach  nida  (arachne,  spider). 

Garden-spider,   taran'tula,   bird-spider,  trap-door  spider,  mite, 
tick,  king-crab  or  horseshoe  crab. 

BRANCH  XIII.   CHORDATA  (chorda,  a  cord,  referring  to  the 

notochord) 

SUBBRANCH  I.   Adelbchcr'  da.     Class  Adelochorda. 
SUBBRANCH  II.   Urochor'da.     Class  Urochord- 

Sea-squirts,  Tunica'ta,  Ascid'ians. 
SUBBRANCH  III.   Vertebra  ta  (vertebratus,  jointed). 

Division  A.   Acra'nia  (a,  without;  cranion,  skull).    Class  Lepto- 

cardii. 

Lancelet  (BranchiSs'toma  =  Amphiox'us). 
Division  B.  Crania'ta. 


250  ANIMAL  FORMS 

Class  I.   Cyclostbm'ata  (cyclos,  circle ;  stoma,  mouth). 
Hagfishes,  lamprey. 

Class  II.   Pis'ces  (piscis,  fish). 

Shark,  skate  or  ray,  lung-fish,  sturgeon,  garpike,  catfish,  horned 
pout,  bullhead,  carp,  dace,  chub,  minnow,  eel,  herring,  shad, 
salmon,  trout,  pike,  stickleback,  blindfish,  sea-horse,  mullet, 
flying-fish,  perch,  darter,  sunfish,  sea-bass,  mackerel,  snapper, 
grunt,  weakfish,  bluefish,  rose-fish,  gurnard,  sculpin,  codfish, 
flounder,  angler. 

Class  III.   Amphibia  (amphi,  double;  bios,  life). 

Siren,  mud-puppy,  water-dog,  tiger  salamander,  axolotl,  toad, 
frog,  tree-frog. 

Class  IV.  Reptil'ia  (reptans,  creeping). 

Skink,  "  glass-snake,"  swift,  chameleon,  horned  toad,  Gila  mon- 
ster, blacksnake,  grass-snake,  milk-snake,  rattlesnake,  copper- 
head, water-moccasin,  soft-shell  turtle,  snapper,  painted  turtle, 
box-turtle,  leather-turtle,  loggerhead,  hawkbill,  crocodile,  alli- 
gator. 

Class  V.  A'ves  (avis,  bird). 

Ostrich,  loon,  grebe,  auk,  murre,  puffin,  gull,  tern,  petrel,  alba- 
tross, cormorant,  pelican,  duck,  goose,  swan,  heron,  bittern, 
crane,  rail,  mud-hen  or  coot,  snipe,  woodcock,  sandpiper,  kill- 
dee  plover,  quail,  grouse,  wild  turkey,  prairie-chicken,  pigeon, 
dove,  eagle,  hawk,  owl,  turkey-buzzard,  cuckoo,  kingfisher, 
woodpecker,  sapsucker,  swift,  humming-bird,  night-hawk, 
whippoorwill,  crow,  jay,  swallow,  warbler,  thrush,  robin, 
bluebird. 

Class  VI.  Mammalia  (mamma,  breast). 

Duck-mole,  ant-eater,  sloth,  armadillo,  sea-cow,  opossum,  kanga- 
roo, porcupine,  mouse,  rat,  muskrat,  woodchuck,  beaver,  rabbit, 
squirrel,  chipmunk,  prairie-dog,  shrew,  mole,  bat,  whale,  gram- 
pus, dolphin,  porpoise,  zebra,  elephant,  giraffe,  deer,  antelope,, 
goat,  sheep,  horse,  cow,  pig,  buffalo,  bear,  raccoon,  otter,  skunk, 
badger,  wolverene,  sable,  mink,  weasel,  dog,  fox,  wolf,  cat, 
lynx,  panther,  monkey,  ape,  baboon,  gibbon,  orang-utan, 
gorilla,  chimpanzee,  man. 


INDEX 


Acipenser  sturio  (illus.),  162. 

Acorn-barnacle  (illus.),  97. 

Air-bladder,  155. 

Albatross,  212. 

Alligator  mississippiensis  (illus.), 

191. 

Alternation  of  generations,  35. 
Altricial,  208. 
Amoeba  (illus.),  12 ;  structure  and 

ha'bits,  12. 

Amoeba-like  protozoa,  12. 
Amphibian,  development  of,  174 ; 

distribution,  178;  anatomy  and 

habits,  179. 
Amphibious  (amphi,  double ;  bios, 

life),  176. 

Amphioxus  (illus.),  157. 
Amphipod  (illus.),  106. 
Anatomy   (anatemno,  to  cut  up), 

defined,  1. 
Angler  (illus.),  169. 
Angle-worm  (illus.),  55. 
Anguillula  aceti  (illus.),  53. 
Animals,  characteristics  of,  2 ;  sim- 
ple and  complex,  18. 
Animals  and  plants  compared,  2. 
Annelids,  55. 

Anosia  plexippus  (illus.),  125. 
Ant,  128 ;  white  (illus.),  120. 
Anteater,  231. 
Antedon  (illus.),  148. 
Antelope,  239. 


Ant-lion  (illus.),  120. 

Apes,  243. 

Arachnida,  characteristics  of,  133. 

ArchoBopteryx,  201. 

Argynnis  cybele  (illus.),  126. 

Ariolimax  columbianus  (illus.),  81. 

Armadillo,  231. 

Arthropods,  general   features   of, 

93. 

Ascidian  (illus.),  152. 
Asexual  reproduction,  31. 
Asterias  ocracea  (illus.),  141. 
Astrophyton  (illus.),  143. 
Auk,  209. 
Axolotl,  183. 

Back-swimmer,  121. 
Balancers,  122. 
Band-worm  (illus.),  70. 
Barnacles  (illus.),  96. 
Bascanion  constrictor  (illus.),  187. 
Basket-star  (illus.),  141.     . 
Bass,  166. 

Bat,  brown,  234;  red,  234. 
Bear,  239. 
Beaver,  232. 
Bees,  238. 
Beetles,  124. 

Bell  animalcule  (illus.),  16. 
Bilateral  symmetry,  44. 
Biology  (bios,  life;    logos,  a  dis- 
course) defined,  1. 

251 


252 


ANIMAL  FORMS 


Birds,  characteristics  of,  201 ;  anat- 
omy of,  204 ;  habits  of,  207. 

Bird  spider,  136. 

Bittern,  215. 

Black-snake  (illus.),  193. 

Blastula  (illus.),  21. 

Bombus  (illus.),  130. 

Bony  fish  (illus.),  160. 

Botany,  defined,  1. 

Brachiopod  (illus.),  70. 

Bradypus  tridactylus  (illus.),  229. 

Branchiostoma  californiense  (il- 
lustration), 157. 

Branchipus  (illus.),  94. 

Brittle-star  (illus.),  141;  regenera- 
tion of,  145. 

Bugs,  121. 

Bumblebee  (illus.),  130. 

Butterflies,  125. 

Buzzard,  219. 

Byssus,  77. 

Caddis-fly,  119. 

Calcolyntlius  primigenius  (illus.), 

25. 

Calypte  anna  (illus.),  224. 
Cancer  productus  (illus.),  104. 
Caprella  (illus.),  106. 
Carapace,  of  Crustacea,  95,  99 ;  of 

turtle,  188. 
Carp,  163. 

Cat,  domestic,  239 ;  wild,  239. 
Catfish,  163. 
Cell  (cella,  a  little  room),  7 ;  shape 

and  size,  7;  typical  (illus.),  8. 
Centipeds  (illus.),  111. 
Cephalopod  (illus.),  87. 
Cephalothorax,  99. 
Cercopithecus  (illus.),  244. 
Cervus  canadensis  (illus.),  238. 
Cestode  (illus.),  50. 
Cete,  234. 


Cheiroptera,  234. 

Chelonia,  188. 

Chinch-bug,  122. 

Chiton  (illus.),  82. 

Chologaster  avetus  (illus.),  164 ;  C. 
agassizi  (illus.),  164. 

Chordate,  characteristics  of,  151. 

Chordeiles  virginianus  (illus.), 
222. 

Chub,  163. 

Cilium  (cilium,  an  eyelash),  16. 

Circulatory  system,  use  of,  4. 

Clam  (illus.),  72 ;  anatomy,  72,  78 ; 
rock-  and  wood-boring,  75. 

Clitellum,  58. 

Coccyges,  220. 

Cockroach,  118. 

•£oelenterate,s,  general  character- 
istics of,  1&* 

Coleoptera  (illus.),  124. 

Columbse,  210. 

Complex  animals,  characteristics 
of,  18. 

Compound  eyes,  109. 

Coot,  215. 

Copperhead,  193. 

Corals  (illus.),  41. 

Cormorant,  212. 

Correlation  of  function  and  struc- 
ture, 6. 

Cottontail,  233. 

Courting  colors,  203. 

Crab,  hermit  (illus.),  102 ;  cancer 
(illus.),  103;  rock  (illus.),  104; 
fiddler,  104. 

Crane  (illus.).  215. 

Crayfish  (illus.),  101. 

Cricket,  118. 

Crinoid  (illus.),- 143. 

Crocodile,  190. 

Crocodilia  (illus.),  190. 

Crotalus  adamanteus  (illus.),  222. 


INDEX 


253 


Crustacea,  93 ;  anatomy  of,  98,  107 ; 

multiplication  of,  98,  110. 
Cteniza  (illus.),  137. 
Cuckoo,  220. 
Cucumaria  (illus.),  146. 
Cuticle,  14. 
Cuttlefish,  87. 
Cutworm,  112. 
Cyclops  (illus.),   95;  anatomy  of, 

98. 
Cyclostomes,  157. 

Daddy-long-legs,  130. 

Decapods,  102. 

Deer,  222. 

Dendrostoma  (illus.),  68. 

Devil-fish  (illus.),  87. 

Didelphys  virginiana  (illus.),  231. 

Diemyctylus  torosus  (illus.),  182. 

Digestive  tract,  use  of,  3. 

Diptera  (illus.),  119. 

Division  of  labor,  21. 

Dog,  223. 

Dolphin,  221. 

Dove,  205. 

Dragon-fly  (illus.),  118. 

Duck-mole  (illus.),  216. 

Ducks  (illus.),  200. 

Eagle,  golden,  205;  bald  (illus.), 
205. 

Earthworm  (illus.),  55;  anatomy, 
55 ;  distribution,  59. 

Echinoderms,  141 ;  locomotor  sys- 
tem, 146 ;  development  of,  150. 

Ecology,  1. 

Eel,  163. 

Egg,  fertilization  of,  20,  21. 

Elk  (illus.),  222. 

Encystment  of  protozoa,  13. 

Epialtus  productus  (illus.),  104. 

Epidella  squamula  (illus.),  49. 


Eretmochelys     imbricata    (illus.), 

195. 

Esox  (illus.),  165. 
Euglena  (illus.),  17. 
Eurypelma  lentzii  (illus.),  136. 

Fairy  shrimp  (illus.),  94. 

Felis  concolor  (illus.),  242. 

Ferse,  239. 

Firefly,  124. 

Fish,  general  characters  of,  154; 

respiration,  155 ;  anatomy,  168  ; 

breeding  habits,  171. 
Fishmoth,  117. 
Fish- worm  (illus.),  55. 
Flagellum  (flagellum,  a  whip),  14. 
Flea,  122. 
Flicker,  221. 

Flies,  122 ;  development  of,  123. 
Flounders,  168. 
Fox  (illus.),  241. 
Frog,  178. 

Cfammarus  (illus.),  106. 

Gallime,  217. 

Ganglion  (ganglion,  a  swelling),  a 
swelling  of  the  nerve-cord  due 
to  the  accumulation  of  nerve- 
cells,  79. 

Ganoidea,  161. 

Garpike,  161. 

Garter-snake,  193. 

Gasteropod  (illus.),  80;  anatomy  and 
physiology,  81. 

Gastric  mill  (illus.),  107. 

Gastrula  (diminutive  of  gaster, 
stomach),  21. 

Geese,  213. 

Gelasimus  (illus.),  104. 

Gephyrean  worms  (illus.),  67. 

Gila  monster  (illus.),  193. 

Glass-snake,  191. 


254 


ANIMAL  FORMS 


Glires,  232. 

Gnat,  122. 

Gonionemus  vertens  (illus.),  34. 

Goose  barnacle  (illus.),  96. 

Gordius,  54. 

Gorilla  (illus.),  245. 

Grasshopper  (illus.),  117. 

Grebe,  209. 

Green  gland,  108. 

Grouse,  218. 

Grua  americana  (illus.),  212. 

Gull,  211. 

Habitat  (habitare,  to  dwell),  45. 

Hagfish,  157. 

Halicetus  leucocephalus  (illus.), 
220. 

Haliclystus  (illus.),  39. 

Harvestman,  135. 

Hawks,  219. 

Helix  (illus.),  81. 

Heloderma  suspeclum  (illus.),  192. 

Hemiptera,  121. 

Heptacarpus  brevirostris  (illus.), 
101. 

Hermit-crab  (illus.),  102. 

Herodines,  215. 

Herons,  215. 

Herring,  163. 

Homo  sapiens,  245. 

Honey-bee,  130. 

Hoofed  animals,  237. 

Horned  toads,  192. 

Hornet,  132. 

Horse-fly  (illus.),  119. 

Horsehair-snake,  54. 

Horseshoe-crab  (illus.),  139. 

Humming-bird  (illus.),  222. 

Hydra,  structure  of,  29 ;  multipli- 
cation of,  31 ;  regeneration  of, 
51. 

Hydractinia  (illns.),  36,  103. 


Hydranth,  33. 

Hydrozoa,  34 ;  regeneration  of,  51. 
Hymenoptera  (illus.),  128. 
Hystrix  cristata  (illus.),  233. 

Incubation  (incumbo,  to  rest  upon), 

207. 

Infusoria,  17. 
Insectivora,  234. 
Insects,   numbers,   114;    anatomy, 

115 ;  respiration,  117. 
Isopod  (illus.),  101. 

Jelly-fish,  of  Hydrozoa,  33 ;  of  Scy. 

phozoa,  37. 
Julus  (illus.),  112. 

Kangaroo,  232. 
Katydid,  117. 
Keyhole-limpet,  82. 
King-crab  (illus.),  139. 
Kingfisher,  221. 

Lacertilia,  185, 
Lamellibranch  (illus.),  72. 
Lamellirostres,  213. 
Lamprey  (illus.),  157. 
Lamp-shell  (illus.),  70. 
Lancelet  (illus.),  157. 
Lasso-cell,  30. 
Leeches  (illus.),  63;    haunts  and 

habits,  64. 
Lemur.  243. 
Lepas  (illus.),  99. 
Lepidoptera  (illus.),  125. 
Lepomis  megalotis  (illus.),  167. 
Leptoplana  (illus.),  45. 
Lice,  122. 
Life  histories   and   race  histories, 

27. 

Limicolae,  216. 
Limulus  polypJiemus  (illus.),  139. 


INDEX 


255 


Littorina,  habits  of,  83. 

Liver-fluke,  49. 

Lizard  (illus.),  185. 

Lobster,  102. 

Locust  (illus.),  117. 

Long-horned  borer  beetle  (illus.), 

124. 

Longipennes,  211. 
Loon,  209. 

LopTiius  piscatorius  (illus.),  169. 
LopTiortyx  californicus  (illus.),  217. 
Lumbricus  terrestris  (illus.),  55. 
Lung-fish,  160. 
Lynx  (illus.),  241. 

Macrobdetta  (illus.),  63. 

Macrocheira,  222. 

Mammals,  characteristics  of,  225; 
anatomy,  226 ;  classification,  228. 

Man,  245. 

Many-celled  animals,  11. 

Marsupialia,  231. 

Marsupium  (marsupium,  a  purse 
or  bag),  232. 

May-fly,  118. 

Megaptera  versabilis  (illus.),  235. 

Mesenteric  filaments,  40. 

Messmates,  defined,  48. 

Metamorphosis,  retrograde,  99 ;  in- 
complete, 126 ;  complete,  128. 

Metazoa,  defined,  11. 

Mice,  233. 

Millipeds  (illus.),  111. 

Minnow,  163. 

Mite  (illus.),  138. 

Mole,  234.  ^ 

Mollusks,  general  characters  of,  72. 

Molt,   of    Crustacea,   99,    110;    of 
birds,  202. 

Monarch  butterfly  (illus.),  125. 

Monkeys,  243. 

Morphology,  defined,  1. 
39 


Mosquito,    122;  development    of, 

123. 

Moths  (illus.),  125. 
Mud-hen,  215. 
Mud-puppy  (illus.),  .178. 
Multiplication  of  animals,  5. 
Muscle-cell  (illus.),  7. 
Muscular  system,  use  of,  4. 
Mussel,  77. 

My  sis  americana  (illus.),  100. 
Mytilus  edulis  (illus.),  77. 

Nauplius  (illus.),  99. 
Nematoda  (illus.),  52. 
Nemertean  worm  (illus.),  70. 
Nereis  (illus.),  59. 
Nerve-cell  (illus.),  7. 
Nettle-cell  (illus.),  30. 
Neuroptera,  118. 
Newt  (illus.),  178. 
Night-hawk  (illus.),  222. 
Notochord,  151. 
Nucleus  (illus.),  9. 

Octopus punctatus. (illus.),  87. 
Oligochaetes,  59. 

Operculum  (operculum,  a  lid),  87. 
Opossum  (illus.),  231. 
Opossum-shrimp  (illus.),  100. 
Orang-utan  (illus.),  243. 
Orb- weaving  spider,  136. 
OrnithorJiynchus  paradoxus  (illus- 
tration), 230. 
Orthoptera,  117. 
Ostrich  (illus.),  209. 
Oyster,  77. 
Owls,  219. 

Pagurus  bernhardus  (illus.),  103. 
Paludicolae,  215. 
Pandorina  (illus.),  19. 
Panther  (illus.),  241. 


ANIMAL  FORMS 


Paramcecium  (illus.),  15. 

Parapodium  (para,  alongside  of; 
pous,  foot),  60. 

Parasitism,  48. 

Passeres,  223. 

Pelican  (illus.),  212. 

Pelicanus  erythrorhynchus  (illus.), 
212. 

Perca  flavescens  (illus.),  155. 

Perch,  166. 

Perching  birds,  223. 

Peripatus  eiseni  (illus.),  111. 

Pheasant,  217. 

Physalia  (illus.),  37. 

Physiology  (physis,  the  nature  of 
a  thing ;  logos,  a  discussion),  1. 

Pici,  221. 

Piddock  (illus.),  75. 

Pigeons,  218. 

Pike  (illus.),  164. 

Pill-bug  (illus.),  105. 

Pineal  gland,  198. 

Planaria  (illus.),  45. 

Plants  and  animals,  differences  be- 
tween, 1. 

Plants,  characteristics  of  higher,  2. 

Plant-lice,  122 ;  in  ant-nests,  129. 

Plastron,  188. 

Plover,.killdee,  216. 

Polychjetes,  59 ;  sedentary  (illus.), 
61 ;  development  of,  63. 

PolynoR  brevisetosa  (illus.),  61. 

Polyzoa  (illus.),  68. 

Porcellio  scdber  (illus.),  105. 

Porcupine  (illus.),  232. 

Porpoise,  234. 

Portuguese  man-of-war  (illus.),  36. 

Prairie-dog,  233. 

Prawn  (illus.),  100. 

Precocial  birds,  208. 

Primates,  243. 

Proboscis,  of  flatworms,  46. 


Procyon  lotor  (illus.),  240. 
Protoplasm  (protos,  first;  plasma, 

anything  molded),  9  ;  structure 

of,  10. 
Protozoa,    11;    characteristics  of, 

17;  colonial,  19. 
Pugettia  richii  (illus.),  104. 
Pulsating  vacuoles,  16, 17. 
Pygopodes,  209. 

Quail  (illus.),  217. 

Rabbit,  233. 

Raccoon  (illus.),  240. 

Race  histories  and  life  histories,  27. 

Radial  symmetry,  44. 

Rail,  215. 

Rain-crow,  220. 

Raptores,  219. 

Rat,  house-,  233 ;  wood-,  233  ; 
musk-,  233. 

RatitjB,  209. 

Rattlesnake  (illus.),  191. 

Recognition-marks,  203. 

Regeneration,  51. 

Reproduction,  sexual  and  asexual, 
5,32. 

Reptiles,  general  characteristics, 
185 ;  distribution,  191  ;  anat- 
omy, 195. 

Retrograde  metamorphosis,  99. 

Rotifer  (illus.),  66. 

Ruminant,  237. 

Sabella,  62. 

Salamander  (illus.),  176;  distribu- 
tion, 178;  structure  of,  179. 
Salmon,  163. 
Sand-dollar  (illus.),  142. 
Sandhopper  (illus.),  105. 
Sandpiper,  216. 
Sapsucker,  221. 


INDEX 


257 


Sarcoptes  scabei  (illus.),  138. 

Scelophorus  undulatus  (illus.),  185. 

Scorpion  (illus.),  134. 

Scyphozoa,  37 ;  development  of,  38. 

Sea-anemone,  40. 

Sea-cucumber  (illus.),  143  ;  regen- 
eration of,  145. 

Sea-lily  (illus.),  143. 

Sea-mat  (illus.),  68. 

Sea-urchin  (illus.),  141. 

Sea-squirt  (illus.),  152. 

Sedentary  life,  effect  of,  62. 

Segments,  of  worms,  55 ;  of  arthro- 
pods, 94. 

Segmented  worms,  55. 

Serpentes,  186. 

Serpent-star  (illus.),  141. 

Serphus  dilatatus  (illus.),  122. 

Serpula  (illus.),  62. 

Serpulids,  62. 

Seta,  55. 

Sexual  reproduction,  32. 

Shark  (illus.),  159. 

Shell-gland,  108. 

Shipworm,  75. 

Shrew,  234. 

Shrimp,  fairy,  94 ;  opossum  (illus.), 
100. 

Silk-moth  (illus.),  127. 

Silver-spot  butterfly  (illus.),  126. 

Simple  animals,  characteristics  of, 
18. 

Single-celled  animals,  11. 

Sinus,  blood,  78. 

Siren  (illus.),  178. 

Slug  (illus.),  80.  .    •/ 

Snail,  common  (illus.),  80;  arma- 
dillo (illus.),  82 ;  naked  (illus.),  82. 

Snakes,  186 ;  distribution  of,  193. 

Snipes,  216. 

Somateria  dresseri  (illus.),  214. 

Species,  origin  of,  91. 


Sperm-cell,  20. 

Sphenodon  punctatus  (illus.),  199. 

Spicule,  of  sponge  (illus.),  26;  of 
coral,  42. 

Spiders,  organization  of,  135 ;  hab- 
its, 136. 

Spinnerets,  135. 

Spiny-rayed  fishes,  166. 

Sponge,  development  of  (illus.), 
21;  distribution,  22;  shape  and 
structure,  23. 

Spontaneous  generation,  54. 

Springtail,  117. 

Squalus  acanthias  (illus.),  159. 

Squash-bug,  122. 

Squid  (illus.),  87. 

Squirrels,  233. 

Starfish  (illus.),  140  ;  regeneration, 
145;  structure,  146. 

Steganopodes,  212. 

Stickleback,  164. 

Structure  and  function,  correla- 
tion of,  6. 

Strongylocentrotus  purpuratus  (il- 
lustration), 144. 

Struthio  camelus  (illus.),  210. 

Sturgeon  (illus.),  161. 

Sunfish  (illus.),  166. 

Symmetry,  radial  and  bilateral,  44. 

Swan  (illus.),  213. 

Swift,  222. 

Tcenia  solium  (illus.),  51. 

Tapeworm  (illus.),  50;  develop- 
ment, 51 ;  in  relation  to  regen- 
eration, 51. 

Tarantula  (illus.),  137. 

Teeth,  use  of,  2. 

Teleostei,  160. 

Termites  (illus.),  120. 

Tern,  211. 

Terrapene  Carolina  (illus.),  189. 


258 


ANIMAL  FORMS 


Thousand-legged  worms  (illus.), 
111. 

Threadworms  (illus.),  52. 

Thysanura,  117. 

Tick,  138. 

Tiger  salamander,  178. 

Toad  (illus.),  178. 

Trap-door  spider  (illus.),  137. 

Trematode  (illus.),  48;  develop- 
ment, 51. 

Trichina  spiralis  (illus.),  53. 

Trigger  hair,  31. 

Turkey,  218. 

Turtles,  188;  structure,  189;  dis- 
tribution, 194. 

TypJilichthys  sulterraneus  (illus.), 
163. 

Ungulata,  237. 

Vacuole,  pulsating,  16 ;  use  of,  17. 
Velum  (illus.),  35. 
Vespa,  nest  of  (illus.),  131. 
Vinegar  eel  (illus.),  53. 
Volvox  (illus.),  19;  multiplication 

of,  20. 
Vertebrates,  characteristics  of,  145 . 

classification,  143. 


Vorticella  (illus.),  16. 

Vulpes  pennsylvanicus  (illus.),  241. 

Wasps,  128 ;  habits  of,  131. 
Water-boatman,  121. 
Water-bug  (illus.),  121. 
Water-dog  (illus.),  178. 
Water-flea,  95. 
Whale,    humpback    (illus.),    235; 

sperm,  236. 
Whale  lice,  107. 
Wheel-animalcule  (illus.),  66. 
Wheel- weaving  spiders,  136. 
Whippoorwill,  222. 
White  ant  (illus.),  120. 
Wood-beetle  (illus.),  124. 
Woodchuck,  233. 
Woodcock,  216. 
Woodpeckers,  221. 
Worms,  general  characters  of,  44° 

classification,  45. 

Yellowhammer,  221. 
Yellow-jacket,  132. 

Zirphcea  crispata  (illus.),  76. 
Zoology,  1. 
Zoophyte,  29. 


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